Bump WP tested-up-to version 5.8

[jeffpaul]

Description of the Change

@phpbits helped test and validate that things work as expected on WP 5.8, so this PR bumps the "tested up to" value to 5.8 and credits @phpbits for his testing.

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:

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

Applicable Issues

Closes #71.

Changelog Entry

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