Docs: "Upgrade support" is confusing and doesn't mention a feature gate

[m1kola]

I think Upgrade support doc requires some rewriting because:

1. It doesn't mention that the semver upgrades require the ForceSemverUpgradeConstraints feature gate which gives a false impression it is a feature which is enabled by default.
2. The flow of the document needs improving: going through the doc it is not clear whether SelfCertified and CatalogProvided upgrade constraint policies work with both legacy upgrade edges and server upgrade edges or not.