Switch phpunit configuration condition to PHP version-based

Author: leith

.github/workflows/phpunit.yml

@@ -44,12 +44,10 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
+      # Fixes any pubkeys failure (add a `composer diagnose` step to debug if necessary)
       - name: "Composer force self-update"
         run: "composer self-update"
 
-      - name: "Composer diagnostics"
-        run: "composer diagnose"
-
       - name: "Install lowest dependencies"
         if: ${{ matrix.dependencies == 'lowest' }}
         run: "composer update --prefer-lowest --no-interaction --no-progress"
@@ -58,10 +56,10 @@ jobs:
         if: ${{ matrix.dependencies == 'highest' }}
         run: "composer update --no-interaction --no-progress"
 
-      - name: "Tests (lowest)"
-        if: ${{ matrix.dependencies == 'lowest' }}
+      - name: "Tests (PHPUnit 9)"
+        if: ${{ matrix.php-version <= '8.0' }}
         run: "vendor/bin/phpunit --configuration phpunit9.xml.dist"
 
-      - name: "Tests (highest)"
-        if: ${{ matrix.dependencies == 'highest' }}
+      - name: "Tests (PHPUnit 10+)"
+        if: ${{ matrix.php-version >= '8.1' }}
         run: "vendor/bin/phpunit"