Add timestamp annotation in tidbcluster statefulset (#1875)

[sre-bot]

cherry-pick #1875 to release-1.1

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What problem does this PR solve?

fixes #1846

When .Values.admission.validation.pods is enabled, the process of upgrading and scaling for pd and tikv would be controlled by the admission webhook.

And the admission webhook would face the problem that, as webhook would deny the request for deleting the pod if the tikv or the pd is not ready to be deleted, the interval duration between each 2 deleting request from sts controller would be much longer. This would cause the timeout error in our CI test. This request is to add the timestamp in each syncing in sts annotations to avoid this problem by adding the tidbcluster sts into the sts controller queue.

Does this PR introduce a user-facing change?:

Fix the timeout error when .Values.admission.validation.pods is true during tikv upgrading.