Test against WP 5.8

[jeffpaul]

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

Describe the solution you'd like

• test ElasticPress on WP 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, otherwise use "Plugin asset/readme update" action to update "tested up to" version on .org repo

Designs
n/a

Describe alternatives you've considered
none

Additional context
Related: #1708, #2124.

[felipeelia]

@jeffpaul after the work we had on #2211, can we consider the three first [ ] done, or did we have any substantial change in WP Core that would require us to test it again? Thanks!

[jeffpaul]

@felipeelia it would probably be worth running tests locally on a 5.8 RC install just to double check things pass as expected, otherwise I agree the testing in #2211 probably suffices for functional testing.

[felipeelia]

In fact, the new widget screen broke part of our WP Acceptance tests. As we've addressed it in #2281, I'm closing this one now. Thanks, @jeffpaul!

[jeffpaul]

@felipeelia if this is tested and good, will still want to update the README.md reference as well.