Record only the latest version for each action in `./.github/workflows/dummy.yml`

[raboof]

gateway.py currently expects ./.github/workflows/dummy.yml to contain only one line per action: if there are multiple, update_actions, it will add an expiration all but one of the 'new' entries as well (like in 08ba8a1)

I'd propose to only record the 'newest' released version in ./.github/workflows/dummy.yml, and use the sha # tag notation supported by dependabot for convenience.

Happy to PR, but I wanted to put this idea up for feedback before going ahead and doing it :)

[raboof]

Actually there's a second reason not to want the tag as a separate entry in there - we want to have SHA hashes everywhere, anyway, right?

[raboof]

@assignUser shared via slack: the additional entries are currently being created from actions.yml.

We probably want to adapt that process to only include the 'latest' hash/version in dummy.yml - but for that we probably want to support tag comments first? I made a first step towards supporting those in #169