*: better handle sysvar upgrades from older versions (#31583)

[ti-srebot]

cherry-pick #31583 to release-5.4
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/32997

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/32997:release-5.4-22f4c33d4007

---

What problem does this PR solve?

Issue Number: close #31538

Problem Summary:

In v5.4 the GetGlobalSysVar() function returns an error if loading the value of a sysvar fails validation (in prior versions; no validation was performed at all, so this can be seen as a regression of #30293 ).

While this could be fixed by adjusting SHOW VARIABLES to handle these errors, it seems easier if we consistently handle upgrade issues in the GetGlobalSysVar() function in a determinstic way: replacing the default value if validation fails.

This also adds tests around system variable upgrades, which unfortunately were lacking previously.

This effectively reverts #31566 because it handles the issue in a different part of the code.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility
  Minor incompatibility: In 5.4 (and only 5.4) it was possible that some noop sysvars permitted incorrect values to be set. These now return an error again as in 5.3 and below.

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@morgo you're already a collaborator in bot's repo.

[morgo]

5.4 has its own fix for this (it was more of a workaround). So we can close this cherry-pick.