bump TiDB Operator to v1.6.0-beta.1, TiDB to v7.5.1

[csuzhangxc]

What problem does this PR solve?

What is changed and how does it work?

only bump TiDB to 7.5.1 as v8.0.0 not has not beed released yet

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.

[ti-chi-bot]

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[csuzhangxc]

/run-all-tests

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.16%. Comparing base (14d9ab4) to head (cfa6bee).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5591      +/-   ##
==========================================
+ Coverage   61.49%   64.16%   +2.67%     
==========================================
  Files         235      246      +11     
  Lines       30335    34228    +3893     
==========================================
+ Hits        18653    21962    +3309     
- Misses       9813    10323     +510     
- Partials     1869     1943      +74     

Flag	Coverage Δ
e2e	33.30% <ø> (?)
unittest	61.48% <ø> (-0.01%)	⬇️

[csuzhangxc]

/run-all-tests

[HuSharp]

/run-pull-e2e-kind

[HuSharp]

/run-pull-e2e-kind-tikv-scale-simultaneously

[csuzhangxc]

/run-pull-e2e-kind

[csuzhangxc]

/run-pull-e2e-kind