Used tag creation timestamp for tag processing

[shankarinece]

Used tag event creation timestamp rather than the associated commit's timestamp. Partially relates to JENKINS-57772.

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[ryancurrah]

Related issue: https://issues.jenkins.io/browse/JENKINS-57772

[nfalco79]

Source branch seems to be deleted and review / comment can not be placed.
Anyway this PR impact cloud events where tag and annotated tag timestamp are valued correctly.