automate releases (v5)

[mweberxyz]

Given that this repo regularly has both 6.x and 5.x releases, and the 5.x branch gets significantly more traffic from npm, it would be best to extend the release workflows to support both branches.

I also realize it was a mistake in #3089 is to assume that the current release always immediately succeeds the last published release (ie- using releases[0] as the previous_tag_name in the calls to generateReleaseNotes) which means a 6.x release following a 5.x release (or vice versa) will wreck the release notes.

@mcollina thoughts? I think it would be easier to take care of both of these tasks at the same time, otherwise let me know and I'll pull out the secondary concern into it's own issue.

[mcollina]

go ahead and send a PR over!