Handle branch out-of-support

[gavin-norman-sociomantic]

Currently, when releases a major branch that is out-of-support, the release tool's action is to make a normal minor release. The tool has the information required to detect that the branch is out-of-support, though, and could perform the extra steps required in that situation. Namely:

• Send a special email entitled [OutOfSupport] repo vY.x.x.
• Change the repo's default branch.
• Delete the out-of-support major branch and all minor branches for the same major.
• (Update the list of supported branches in the repo's README.)

[gavin-norman-sociomantic]

Of course, when doing this, the usual logic of creating the minor tracking branches can be omitted, as well.