We should consolidate and update documentation around upgrade/downgrade, FCV, feature flag, and multiversion code, process, policy, and testing in the Wiki and code, and deprecate any old updated docs

For example:

• "JS tests tagged with this feature flag need to be converted to a “requries_fcv_XY” tag to ensure they’re run on all build variants in Evergreen. (The requires_fcv_XY tag is enforced as of SERVER-55858)" The above point may need to removed from the wiki

See https://docs.google.com/document/d/1ZkWjvl4ThuD85dzlSfCys2p2MiGuI0C4mvUYHWIJoJQ/edit#bookmark=id.tnvqrpaxbndk for docs to audit

• Also update server best practice docs so that future server engineers only support full FCV/Binary downgrade for LTS->LTS

• Update comments/documentation to emphasize best practices for adding functionality to setFCV command. In particular emphasize this point