Update/wp version to 5.8

[jeffpaul]

Description of the Change

@phpbits helped test WP 5.8 compatibility and found that #97 is needed, so this PR bumps the "tested up to" value to 5.8 once we get 97 merged in.

Alternate Designs

n/a

Benefits

Ensures we properly state our tested-up-to version on GitHub and WP.org.

Possible Drawbacks

none identified.

Verification Process

@phpbits tested the plugin locally.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Relates to #95.

Changelog Entry

### Changed
- Bump WordPress "tested up to" 5.8 (props @phpbits, @BBerg10up).