Add PHP 8.4 and remove PHP 8.0 in automated PHP unit tests

[daledupreez]

Related to #4214

Changes proposed in this Pull Request:

This PR iterates on #4214 by increasing the number of parallel runs that we allow for the PHP tests CI workflow, adding PHP 8.4 as the latest PHP version, and drops PHP 8.0 from our tested versions.

Testing instructions

• Verify that the CI tests continue to pass
• The runs should complete slightly faster due to increasing the parallelism

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• Covered with tests (or have a good reason not to test in description ☝️)
• [N/A] Tested on mobile (or does not apply)

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Post merge

• [N/A] Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• [N/A] Added the needs docs label (or does not apply)
• [N/A] Included this PR in the Release Thread scope (or does not apply)