Owner: Fix gcttl bug (#1861)

[ti-chi-bot]

This is an automated cherry-pick of #1861

What problem does this PR solve?

#1839

What is changed and how it works?

Modify the calculation method of MinGCSafePoint （the lower bound of CDC's gcSafePoint）

Before：
MinGCSafePoint = curentTs - gcTTL

After：
MinGCSafePoint = min(curentTs - gcTTL, pdSafePoint)

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Side effects

• Increased code complexity

Related changes

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

Release note

• owner: When tikv_gc_life_time is greater than gcttl, use tikv_gc_life_time to calculate the lower bound of gcSafePoint.

[amyangfei]

/run-all-tests

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.0@1c299a7). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             release-5.0      #1872   +/-   ##
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  Files                  ?        155           
  Lines                  ?      16478           
  Branches               ?          0           
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  Hits                   ?       8881           
  Misses                 ?       6651           
  Partials               ?        946           

[asddongmen]

/lgtm

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[overvenus]

/lgtm

[ti-chi-bot]

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[overvenus]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: a6af59d

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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[asddongmen]

/run-unit-tests