[release-v1.5] Fix deletion case for extension predicates

[timuthy]

How to categorize this PR?

/area quality
/kind bug
/priority normal

What this PR does / why we need it:
This PR fixes the deletion of shoots or their extension resources in the seed once shoots were in a Failed state.

Special notes for your reviewer:

Release note:

An issue has been fixed which caused failed shoot clusters to not be deleted successfully after a retry had been triggered.

[rfranzke]

Maybe I missed it, but why isn't it a 1:1 backport of #2435? This is a different commit now?

[danielfoehrKn]

@rfranzke I think this is the "safer" version (only adding a predicate to make the deletion case working) from there.
Though I am not sure, thinking about it, if it would not be better to backport the exact same commit.

[timuthy]

Well, that's exactly what we discussed here after the suggestion from @ialidzhikov and your response.

[rfranzke]

I see, probably I missed this point, sorry. I'm fine with this PR if you think it makes more sense, but why can't we simply take the same fix that we did for the master branch?

[rfranzke]

/lgtm
/ok-to-test

[timuthy]

@rfranzke I have no strong opinion but know it was confusing to apply a different fix as a back port because it looked differently. Now, I applied the exact same changes we had applied to the master branch.

[rfranzke]

Thanks Tim!