Rename "Unmanaged" to "Self-Managed"

In discussion with TSDR on a recent TSC, it became clear that the name "Unmanaged" has implications we didn't expect, don't think are true, don't desire. It seems to imply to some that "no one is managing the project, it's not being maintained, taken care of" vs what we meant "the release team isn't managing the project's releases". The name "Self-Managed" was proposed as an alternative.

Change the Managed Release docs to use this new term. Get approval from the TSC before merging patch.