Compatibility testing with WordPress 6.6

[vikrampm1]

With the WordPress 6.6 RC out, perform the following:

• E2e tests run only on the latest WP version, so ensure the release is out before running the tests.
• Open PR to bump WordPress "tested up to" to 6.6.
• Bump WordPress minimum supported to 6.1.
• Search codebase for conditionals on minimum version numbers no longer supported that could be removed.
• Check to see if e2e tests pass, if so move the issue to UAT. If e2e tests fail, see if that's an issue with the test or compatibility with WP.
• Check other files where the version needs to be bumped - For this the main thing to look for is the function version_compare and then see if we're checking WordPress versions there or something else (sometimes we check PHP versions for instance or Woo versions). Some plugins also define what WordPress version we support (sometimes as a minimum) in a constant (see Square as an example). There's not always consistent naming on those constants so really the easiest way to find these is to search the codebase for the version string. Search the last few releases to catch them all (so search for 6.3, 6.2, 6.1). Also, suggest looking at the last PR that bumped versions because to copy what was done for those.

Sample Changelog entry should then be:

Dev - Bump WordPress "tested up to" version 6.6.
Dev - Bump WordPress minimum supported version to 6.4.

[dkotter]

Note for any future WP version bumps, our policy at the moment is L-4. This means take the next WordPress version (in this case 6.6) and subtract 4 from that for our minimum (so 6.2 in this case).