Update versioning policy to reflect reality

Currently Versioning says:

> The etcd project maintains release branches for the current version and previous release. For example, when v3.5 is the current version, v3.4 is supported. When v3.6 is released, v3.4 goes out of support.

However, we materially support 2 old versions; currently, we are supporting 3.7, 3.6, and 3.5. So the policy should be updated to reflect that.

Locate the Versioning page containing the quoted release-branch policy and read its surrounding documentation for context. Update the policy to reflect support for the current release and two previous releases, then verify the rendered website states the supported versions accurately.

Written by the indexing model from the issue text.