[fix](dcr) fix cluster updated and add safe shield for scale down fe

[intelligentfu8]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
    restrict only scale down observers.
    1. fe replicas = 1 => scale up replicas=2. scale up. expectation replicas=2
    2. fe replicas =2 => scale down replicas=1 scale down failed. expectation replicas=2 (election number in default =3)
    3. fe replicas=5 => scale down replicas=4 scale down success. expectation replicas=4(election number in default=3)
    4. fe replicas=4=>scale down replicas=2 scale down one pod. expectation replicas=3(election number in default=3)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.
    when scale -down followers will not be allowed.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label