Test against WordPress 6.0

[vikrampm1]

Is your enhancement related to a problem? Please describe.
Once WordPress 6.0 is released, we'll want to test Safe Redirect Manager to see if any incompatibility issues arise.

Describe the solution you'd like

• test Safe Redirect Manager on WordPress 6.0
• 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: #258, #245, #232, #227, #204

[peterwilsoncc]

While testing this against 6.0, I've created #269

Testing against older versions of WP, it appears to have been around for a while so I think this is safe to bump to inidicate 6.0 support. I'll create a pull request.