[Ops] Add official ability to target specific versions in backport

[watson]

Sometimes it's desirable to backport a PR to the previous major without having to also backport it to the previous minor. Today this is technically possible using the label auto-backport combined with specific v* labels. However, this label is technically deprecated and it's recommended to use the backport:* labels instead. But none of these support this use-case (contrary to what I originally believed, backport:prev-MAJOR will backport to the previous major AND any open branch in between).

So I propose a new label along the lines of backport:selected-versions or backport:specific-versions or something shorter - I don't like it's so long.

Background

I imagine this is mainly needed when backporting tooling PRs. Here you don't really care about the previous minor because it's so short lived, whereas it's important to keep the previous major in sync with main since it's going to be around for so long.

Example for a situation recently where I needed this, and where I first tried to use backport:all-open, but had to undo it because the backport to 8.9 was causing too many problems: #162464

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[jbudz]

elastic/kibana-github-actions#40 + #190643