Replace deprecated xref: [exclude:] with elixirc_options: [no_warn_undefined:] (#488)

Elixir 1.20 deprecates the project-level `xref: [exclude: ...]` option in favor of
`elixirc_options: [no_warn_undefined: ...]`, which has been the canonical compiler
option since Elixir 1.7. mint requires `~> 1.12`, so the new option is valid across
the entire supported range — no version gating needed.

Behavior is identical: the same modules/MFAs are excluded from undefined-function
warnings. Verified `mix compile --warnings-as-errors` stays clean with castore
(optional) absent.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: patrols

mix.exs

@@ -13,9 +13,9 @@ defmodule Mint.MixProject do
       elixirc_paths: elixirc_paths(Mix.env()),
       deps: deps(),
 
-      # Xref
-      xref: [
-        exclude: [
+      # Suppress "undefined function" warnings for optional/runtime deps.
+      elixirc_options: [
+        no_warn_undefined: [
           {:ssl, :cipher_suites, 1},
           {:public_key, :cacerts_get, 0},
           CAStore