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.