Upgrade coverage.json schema to JSON Schema 2020-12

Draft-07 is from 2018. 2020-12 is the current published draft, supported
by every modern validator (including json_schemer in the dev Gemfile)
and by every IDE that auto-resolves $schema URLs. There is no
compatibility reason to ship a 2018-era contract as the public schema.

Update the meta-schema URI in both schema files (the versioned canonical
and the unversioned alias) and switch the spec assertion from
JSONSchemer.draft7 to JSONSchemer.draft202012. Retitle the README
section to "JSON Schema for coverage.json" so it is searchable for
someone looking for "JSON schema simplecov" rather than for the internal
filename, and tighten the opening paragraph (the second bullet was
redundant with the prose that followed).

Author: sferik

README.md

@@ -1151,14 +1151,11 @@ SimpleCov.formatter = SimpleCov::Formatter::JSONFormatter
 
 > The JSON formatter was originally a separate gem called [simplecov_json_formatter](https://github.com/codeclimate-community/simplecov_json_formatter). It is now built in and loaded by default. Existing code that does `require "simplecov_json_formatter"` will continue to work.
 
-### `coverage.json` schema
+### JSON Schema for `coverage.json`
 
-The `coverage.json` payload is treated as a public contract. Every emitted file carries:
+`coverage.json` is a public contract, described by a JSON Schema (2020-12) so downstream tools can validate it, generate types, or pin to a known shape. Every emitted document carries a top-level `$schema` URL pointing at the versioned canonical, plus a human-readable `meta.schema_version` (`"major.minor"`).
 
-- a top-level `$schema` URL pointing at the exact versioned schema the document conforms to (machine-resolvable, so consumers like IDEs and validators auto-fetch the right contract),
-- a `meta.schema_version` (`"major.minor"`) for human-readable inspection.
-
-JSON Schema (draft-07) is published in the repo. The **versioned canonical** lives at [`schemas/coverage-v1.0.schema.json`](schemas/coverage-v1.0.schema.json) and is immutable per version: long-lived integrations should pin to it. A convenience alias at [`schemas/coverage.schema.json`](schemas/coverage.schema.json) always tracks the latest, and may shift when a new SimpleCov release bumps the schema.
+The **versioned canonical** lives at [`schemas/coverage-v1.0.schema.json`](schemas/coverage-v1.0.schema.json) and is immutable per version, so long-lived integrations should pin to it. A convenience alias at [`schemas/coverage.schema.json`](schemas/coverage.schema.json) always tracks the latest and may shift when a new SimpleCov release bumps the schema.
 
 The schema version is independent of the gem version:
 

schemas/coverage-v1.0.schema.json

@@ -1,5 +1,5 @@
 {
-  "$schema": "http://json-schema.org/draft-07/schema#",
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
   "$id": "https://raw.githubusercontent.com/simplecov-ruby/simplecov/main/schemas/coverage-v1.0.schema.json",
   "title": "SimpleCov coverage.json",
   "description": "Schema for the coverage.json file emitted by SimpleCov's JSONFormatter. Versioned independently of the gem. Non-breaking additions bump the minor segment of schema_version, breaking changes bump the major segment. The versioned file at schemas/coverage-vX.Y.schema.json is the canonical artifact for that version, schemas/coverage.schema.json is a convenience alias for the latest.",

schemas/coverage.schema.json

@@ -1,5 +1,5 @@
 {
-  "$schema": "http://json-schema.org/draft-07/schema#",
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
   "$id": "https://raw.githubusercontent.com/simplecov-ruby/simplecov/main/schemas/coverage.schema.json",
   "title": "SimpleCov coverage.json (latest)",
   "description": "Convenience alias for the latest coverage.json schema. Mirrors schemas/coverage-v1.0.schema.json except for the $id (which is the unversioned URL). For long-lived integrations, pin to the versioned canonical (schemas/coverage-vX.Y.schema.json) so the contract you validate against does not silently shift when a new SimpleCov release bumps the schema.",

spec/formatter/coverage_schema_spec.rb

@@ -20,12 +20,12 @@ def validate_against_schema(document)
     schemer.validate(document).map { |e| "#{e['data_pointer']}: #{e['error']}" }
   end
 
-  it "is itself a valid JSON Schema (draft-07)" do
-    expect(JSONSchemer.draft7.validate(schema_doc).to_a).to be_empty
+  it "is itself a valid JSON Schema (2020-12)" do
+    expect(JSONSchemer.draft202012.validate(schema_doc).to_a).to be_empty
   end
 
-  it "declares draft-07 as its meta-schema" do
-    expect(schema_doc.fetch("$schema")).to eq("http://json-schema.org/draft-07/schema#")
+  it "declares 2020-12 as its meta-schema" do
+    expect(schema_doc.fetch("$schema")).to eq("https://json-schema.org/draft/2020-12/schema")
   end
 
   # The unversioned alias is a convenience pointer to the latest version.