verify and claim compatibility with WordPress 6.3

[openmindculture]

• [x] Install a WordPress 6.3 beta or release candidate.
• [x] Prove that the plugin still work without errors.
• [x] update the "tested up to" meta information everywhere.
• [x] optionally also verify using the latest PHP version
• [x] optionally add a note (in README? as there is no official "tested up to PHP version" field) about latest tested PHP version
• [x] optionally update or add checks for known issues