Test against WordPress 5.9

[jeffpaul]

Is your enhancement related to a problem? Please describe. Once WordPress 5.9 is released, we'll want to test Post Finder to see if any incompatibility issues arise.

Describe the solution you'd like

• [ ] test Post Finder on WordPress 5.9
• [ ] open issues for any incompatibilities noted in testing
• [ ] resolve issues identified in testing
• [ ] bump "tested up to" version
• [ ] if code changes needed due to incompatibilities, ship a plugin release

Designs n/a

Describe alternatives you've considered none

Additional context Related: #34.