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 Simple Local Avatars to see if any incompatibility issues arise.

Describe the solution you'd like

• test Simple Local Avatars 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, 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: #56, #66, #70.

[phpbits]

@jeffpaul I briefly tested the plugin in single and multi-site WordPress installation using version 5.8 and I did not experience any issues. The user avatar is working as expected. Thanks!