GH Actions/test: remove the experimental key from the matrix

... as, at this time, it isn't really needed.

.github/workflows/test.yml

@@ -32,8 +32,6 @@ jobs:
     runs-on: ubuntu-latest
 
     strategy:
-      # Keys:
-      # - experimental: Whether the build is "allowed to fail".
       matrix:
         # The GHA matrix works different from Travis.
         # You can define jobs here and then augment them with extra variables in `include`,
@@ -43,29 +41,25 @@ jobs:
         # The matrix is set up so as not to duplicate the builds which are run for code coverage.
         php_version: ['5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '8.0', '8.1', '8.2']
         cs_dependencies: ['lowest', 'highest']
-        experimental: [false]
 
         include:
           # Experimental builds. These are allowed to fail.
 
           # PHP nightly
           - php_version: '8.3'
             cs_dependencies: 'highest'
-            experimental: true
           # Test against WPCS unstable. Re-enable when WPCS is not in dev for the next major.
           #- php_version: '8.0'
           #  cs_dependencies: 'highest'
-          #  experimental: true
 
           # Test against the next major of PHPCS. Temporarily disabled due to upstream bugs.
           #- php_version: '7.4'
           #  cs_dependencies: 'highest'
           #  phpcs_version: '4.0.x-dev'
-          #  experimental: true
 
     name: "Test${{ matrix.cs_dependencies == 'highest' && ' + Lint' || '' }}: PHP ${{ matrix.php_version }} - CS Deps ${{ matrix.cs_dependencies }}"
 
-    continue-on-error: ${{ matrix.experimental }}
+    continue-on-error: ${{ matrix.php == '8.3' }}
 
     steps:
       - name: Checkout code