Bump operator to v1.2.0-rc.2, tidb to v5.1.0 and DM to v2.0.4

[KanShiori]

What problem does this PR solve?

ref #3999.

run script hack/bump-version.sh, and correct some comments by hand.

What is changed and how does it work?

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

None

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

Merging #4057 (2961f7e) into master (83a2db0) will increase coverage by 9.93%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4057      +/-   ##
==========================================
+ Coverage   61.11%   71.05%   +9.93%     
==========================================
  Files         178      182       +4     
  Lines       18868    19374     +506     
==========================================
+ Hits        11532    13766    +2234     
+ Misses       6195     4482    -1713     
+ Partials     1141     1126      -15     

Flag	Coverage Δ
e2e	57.27% <ø> (?)
unittest	61.14% <ø> (+0.02%)	⬆️