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 Page Ordering to see if any incompatibility issues arise.

Describe the solution you'd like

• [x] test Simple Page Ordering on WordPress 5.8
• [ ] open issues for any incompatibilities noted in testing
• [ ] resolve issues identified in testing
• [x] 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: #46, #65.

[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 plugin is working as expected. Thanks!