Bump minimum WordPress version to at least 6.7 (Yes, “SIX SEVEN”) but ideally 6.9

[westonruter]

There are parts of the codebase which can be improved by updating to WordPress 6.7 (🫲🥴🫱67), for example:

performance/plugins/optimization-detective/tests/class-optimization-detective-test-helpers.php

Line 287 in 7becbba

// TODO: Once WP 6.7 is the minimum-supported version, replace this with WP_HTML_Tag_Processor::set_modifiable_text().

performance/plugins/optimization-detective/tests/class-optimization-detective-test-helpers.php

Line 318 in 7becbba

// TODO: Once WP 6.7 is the minimum-supported version, replace this with WP_HTML_Tag_Processor::set_modifiable_text().

performance/plugins/optimization-detective/tests/class-optimization-detective-test-helpers.php

Line 335 in 7becbba

// TODO: Once WP 6.7 is the minimum-supported version, replace this with WP_HTML_Tag_Processor::set_modifiable_text().

Note the Version Support Policy:

The Performance Lab plugin commits to supporting the latest stable version of WordPress core. With that said, the minimum WordPress version requirement will not be bumped unnecessarily, so realistically the plugin may support additional versions of WordPress core. If or once a feature that is only available in the latest WordPress core stable release is required for a plugin module, the minimum requirement of the plugin will be bumped to that version.

Additionally, as seen in #2353, the unit tests would greatly benefit from being able to use assertEqualHTML which was introduced in 6.9.

Since 6.9 is the most recent stable version of WordPress core, and since we'll be able to reduce technical debt by updating to it, not to mention reduce our testing matrix and maintenance burden (although we don't in reality test in older versions of WP anyway), I suggest we go ahead and bump to 6.9.

Related:

• Minimum version of PHP can be bumped to 7.4 #2340