GH Actions: enable linting and testing against PHP 8.3

[jrfnl]

Summary

This PR can be summarized in the following changelog entry:

• The plugin has no known incompatibilities with PHP 8.3

Relevant technical choices:

• As the PHP 8.3 builds pass and the PHP 8.3 release is expected later this week, the builds are not allowed to fail.
• Update PHP version on which code coverage is run (high should now be 8.3 what with the release this week).

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• N/A If the builds pass, we're good.

[coveralls]

Pull Request Test Coverage Report for Build 6925856769

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 56.274%

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Totals
Change from base Build 6925841856:	0.0%
Covered Lines:	148
Relevant Lines:	263

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💛 - Coveralls