AthenaKernel+CxxUtils: Mixed container trimming workaround.

Scott Snyder requested to merge ssnyder/athena:trimall.CxxUtils-20230417 into 23.0

When trimming a mixed container, allow removing all elements in all the inner containers but the last. Otherwise, we may miss removing elements in dependent containers which have narrower IOVs. That can in turn cause problems with the workarounds needed for the fact that the tracking conditions blithely ignore the lifetime rules for conditions objects.

Also fix a few typos in comments.


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