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 MU Migration to see if any incompatibility issues arise.

Describe the solution you'd like

• [ ] test MU Migration 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 release

Designs n/a

Describe alternatives you've considered none

Additional context none

[jeffpaul]

Note that I updated this from WP 5.3 to 5.4 as the new major release is pending the end of this month.

[jeffpaul]

Note that I'm updating this from testing against WP 5.8 to 5.9.