PHPCS 3.8.0: remove temporary test exclusion for fixer conflict check

[jrfnl]

Rationale

PR #2243 adds a temporary exclusion to the fixer conflict check related to an upstream sniff.

Once the upstream PHPCS sniff has been fixed (PR is open), that exclusion should be removed again.

Note: in the mean time, it is unlikely this issue will cause problems for real-life code as the problem involves a PHP compile error.

References

• Upstream PR: Generic/LowerCaseType: fix potential PHP notice squizlabs/PHP_CodeSniffer#3833
• PR which introduced the exclusion: GH Actions: fix fixer conflict check #2243

Action Checklist

Once a PHPCS version is available which handles this correctly...

• Update the minimum supported PHPCS version in composer.json and update the README and workflows etc.
• Remove the test exception from the fixer conflict check in the GH Actions workflow.