sessionctx: Fix tidb_gc_scan_lock_mode sysvar doesn't show correct default value on new clusters (#25112)

[ti-srebot]

cherry-pick #25112 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25118

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

git push [email protected]:ti-srebot/tidb.git pr/25118:release-5.0-66e68be29658

---

What problem does this PR solve?

Issue Number: close #25100

Problem Summary: When a new cluster is deployed and it has never run GC, the tikv_gc_scan_lock_mode doesn't exist. In this case, select @@global.tidb_gc_scan_lock_mode will return the default value defined in sysvar.go. However, it's inconsistent with the actual default config, so it returns "PHYSICAL" while the actual mode is "LEGACY".

What is changed and how it works?

What's Changed: Changed the default value of sysvar global.tidb_gc_scan_lock_mode to "LEGACY".

Related changes

• Need to cherry-pick to the release branch
  • release-5.0

Check List

Tests

• Manual test (add detailed scripts or steps below)
  • Deploy the cluster by tiup playground and run "select @@global.tidb_gc_scan_lock_mode" immediately, an it returns "LEGACY".

Side effects

• None

Release note

• Fix a bug that a new cluster's "tidb_gc_scan_lock_mode" global variable shows "PHYSICAL" instead of the actual default mode "LEGACY".

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[cfzjywxk]

/merge

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• cfzjywxk
• youjiali1995

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[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 1836f3d

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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[youjiali1995]

/merge