Define support policy for WordPress core versions and plugin modules

[felixarntz]

As stated in https://make.wordpress.org/core/2022/03/07/the-performance-lab-plugin-has-been-released/:

Because the Performance Lab plugin is a collection of potential WordPress core feature modules, the list of modules included may drastically change over time. New modules may be added regularly, while other modules may be removed in a future plugin version once they have landed in a WordPress core release.

Related to the above, specifically the point on module removal, it is essential that we define a policy to ensure users can rely on the plugin's functionality without having to worry that it is suddenly no longer there. In addition, we should formalize the policy for which minimum WordPress core version the plugin will require.

[felixarntz]

I have created a draft PR #240 with documentation of my proposed approach for the policy. At a very high level:

• Always support at least the latest stable WordPress core version. However, the minimum version requirement should only be bumped once a feature from a newer stable WordPress core version is required for a module.
• Never remove (critical functionality from) modules after a core merge without also bumping the minimum version requirement to that WordPress core version where the module was released with.

For more details on these two points, please review the draft PR. Let me know your thoughts and whether this policy is reasonable for you.