ci: drop PHPUnit 8 support

With PHP 7.4 as the minimum, PHPUnit 9 (which requires PHP 7.3+)
covers the entire supported range. This removes the PHPUnit 8 schema
validation step and tightens the Composer constraint accordingly.

Author: GaryJones

.github/workflows/basics.yml

@@ -74,17 +74,8 @@ jobs:
           pattern: ".phpcs.xml.dist"
           xsd-file: "vendor/squizlabs/php_codesniffer/phpcs.xsd"
 
-      # Notes:
-      # - PHPUnit 9.5 (which will be installed in this job) doesn't ship XSD files further back than 8.5.
-      # - PHPUnit 9.3 introduced some new configuration options and deprecated the old versions of those.
-      #   For cross-version compatibility with older PHPUnit versions, those new config options cannot be used yet,
-      #   which is why the PHPUnit 9 validation is done against the PHPUnit 9.2 schema.
-      - name: "Validate PHPUnit config for use with PHPUnit 8"
-        uses: phpcsstandards/xmllint-validate@v1
-        with:
-          pattern: "phpunit.xml.dist"
-          xsd-file: "vendor/phpunit/phpunit/schema/8.5.xsd"
-
+      # Note: PHPUnit 9.3 introduced some new configuration options and deprecated the old versions of those.
+      # For cross-version compatibility, the validation is done against the PHPUnit 9.2 schema.
       - name: "Validate PHPUnit config for use with PHPUnit 9"
         uses: phpcsstandards/xmllint-validate@v1
         with:

composer.json

@@ -28,7 +28,7 @@
 		"php-parallel-lint/php-console-highlighter": "^1.0.0",
 		"phpcompatibility/php-compatibility": "^9",
 		"phpcsstandards/phpcsdevtools": "^1.0",
-		"phpunit/phpunit": "^8 || ^9"
+		"phpunit/phpunit": "^9"
 	},
 	"config": {
 		"allow-plugins": {