ci: Test against the supported PHP floor and ceiling

Run the check matrix on PHP 8.0 and 8.5 rather than the latest two
releases, so continuous integration covers both ends of the range the
package declares support for.

Default the setup action and the build workflow to 8.5, matching the top
of the matrix as the Ruby and Python SDKs do.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RFK47311nWtbd3gHmjdknR

Author: claude

.github/actions/setup/action.yml

@@ -6,7 +6,7 @@ inputs:
   php_version:
     description: The PHP version.
     required: false
-    default: '8.4'
+    default: '8.5'
   install_dependencies:
     description: Install dependencies.
     required: false

.github/workflows/_build.yml

@@ -8,7 +8,7 @@ on:
         description: The PHP version.
         type: string
         required: false
-        default: '8.4'
+        default: '8.5'
     outputs:
       artifact_name:
         description: The artifact name.

.github/workflows/check.yml

@@ -20,7 +20,7 @@ jobs:
         os:
           - ubuntu-latest
         php:
-          - '8.4'
+          - '8.0'
           - '8.5'
         include:
           - os: ubuntu-latest
@@ -44,7 +44,7 @@ jobs:
       fail-fast: false
       matrix:
         php:
-          - '8.4'
+          - '8.0'
           - '8.5'
     steps:
       - name: Checkout
@@ -73,7 +73,7 @@ jobs:
         os:
           - ubuntu-latest
         php:
-          - '8.4'
+          - '8.0'
           - '8.5'
         include:
           - os: ubuntu-latest

README.md

@@ -218,7 +218,7 @@ PHPUnit is configured in `phpunit.xml.dist`.
 ### Requirements
 
 This package supports PHP 8.0 and later.
-Continuous integration exercises the latest two releases, PHP 8.4 and 8.5.
+Continuous integration exercises both ends of that range, PHP 8.0 and 8.5.
 
 ### Publishing