bumped "tested up to" and WP Version to 5.8

[barneyjeffries]

Description of the Change

Tested plugin against WordPress 5.8 and bumped "tested up to" and badge version.

Alternate Designs

Benefits

Possible Drawbacks

Verification Process

• Installed plugin in a local env running 5.8.
• Activated plugin and created a publication, issue statuses, print issue & article statuses
• Create a post and added sections to publication and added posts
• Export XML file
• Add users with 2 x print roles
• Login as user and verify restricted access for production role

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