minor symfony#54188 Add an experimental CI job for PHP 8.4 (stof)

This PR was merged into the 5.4 branch.

Discussion
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Add an experimental CI job for PHP 8.4

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | Relates to symfony#54180
| License       | MIT

This adds the CI job requested in symfony#54180. This PR does not focus on making tests green on 8.4 but the CI job is marked as experimental so that the CI job will still be green (to not block PRs) even when tests are failing. The test report will show failures when looking at the output of the step running tests, which allows moving forward incrementally to make tests green.

Commits
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5f8fc3a Add an experimental CI job for PHP 8.4

Author: stof

.github/workflows/unit-tests.yml

@@ -33,7 +33,8 @@ jobs:
           - php: '8.2'
             mode: low-deps
           - php: '8.3'
-            #mode: experimental
+          - php: '8.4'
+            mode: experimental
       fail-fast: false
 
     runs-on: ubuntu-20.04