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.