Update the milestone during issue report creation for docs repo

[DelazJ]

calling the existing action in the workflow

As you can see in qgis/QGIS-Documentation#9319 and qgis/QGIS-Documentation#9318, the reported version is 3.40 instead of 3.42, meaning that #57421 which is supposed to update the version number during merging didn't work as expected. This PR provides an alternative approach.

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[DelazJ]

Well, this doesn't work: #59061 should have had its milestone/version set to 3.42 but it didn't. As far as I understand, this is due to the way the milestone is set: only PRs that NOT YET have a milestone assigned get one. For years, this would boil down IMHO to the last newly created PR.
I think the logic in pr-auto-milestone action should be modified to tackle only the active (triggering) PR (and not browse all opened?) and thus allow to update its milestone if required.
Would the analysis be correct and the suggestion accepted, I could give it a try (not that I fully understand all the steps there yet, but I can try...).