This commit:
* Adds a proper set of builds against PHP 8.1, 8.2 and PHP 8.3 against low/high WP versions supported for those PHP versions as those PHP versions have been released and should be supported.
* Removes the "experimental" builds against PHP 8.1 and 8.2 as those PHP versions have been released and should be supported.
* Moves the builds against PHP 5.6 from the "complementary" builds section to the "PHP version support" section.
* Moves some comments around to make it more straight forward to understand how this matrix should be read and updated.

I've verified that most WP versions between the lowest and the highest supported version now have at least one build.