Run CI tests on PHP 8.3

[jnoordsij]

Description

Add PHP 8.3 to CI test runs.

Motivation and Context

With a few RC's released and the final release coming up in november, it might be a good thing to start running tests on the new PHP.

See also mollie/mollie-api-php#697.

How Has This Been Tested?

Screenshots (if appropriate):

n/a

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only (no code changes)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[sandervanhooft]

Thanks @jnoordsij !