build: PHP 8 support

[geerteltink]

Enable PHP 8 support.

Not all dev dependencies are ready for PHP 8. Once they are the ignore-platform-reqs can be removed from the phpunit GitHub action.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://www.conventionalcommits.org/
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• build: Changes that affect the build system or external dependencies
• ci: Changes to our CI configuration files and scripts
• docs: Documentation content changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that neither fixes a bug nor adds a feature
• style: Changes that do not affect the meaning of the code
• test: Adding missing tests or correcting existing tests
• revert: Revert previous commit

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information