Test against WordPress 5.8

[jeffpaul]

Is your enhancement related to a problem? Please describe.
Once WordPress 5.8 is released, we'll want to test Distributor to see if any incompatibility issues arise.

Describe the solution you'd like

• test Distributor on WordPress 5.8
• 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: #568, #624, #735.