[Enhance] Discard statefulset rollingUpdate update strategies.

Feature Request

Is your feature request related to a problem? Please describe:
Old discussion issue https://github.com/pingcap/tidb-operator/issues/4130.

I learned from es operator and kafka operator two projects, which all good operator project. They all discard statefulset rollingUpdate update strategies.

ES Operator project related issue:
Use OnDelete instead of RollingUpdate to roll more than 1 pod at a time
It use ondelete strategies instead of sts rollingupdate strategy.

Kafka Operator project related issue and pr:
[Enhancement] Allow scaling down of a pods in the middle of the cluster
Remove StatefulSets from ZooKeeper
It use custom podset crd instead of sts crd.

Describe the feature you'd like:
We should follow the good project, of course take the smallest changes, so i advice that take the ondelete update strategy instead of rollingUpdate strategy and customize tidb operator own update logic.

Start by reviewing the earlier TiDB Operator discussion in issue #4130 and the linked Elasticsearch and Strimzi examples. Identify where StatefulSet rollingUpdate strategies and TiDB Operator update behavior are handled. Done would mean using an ondelete strategy and providing the operator's own update logic, but the issue does not name files or tests.