sessionctx: Fix tidb_gc_scan_lock_mode sysvar doesn't show correct default value on new clusters

[MyonKeminta]

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".

[bb7133]

So the point is, we changed the default value back to 'legacy'(pingcap/docs#5136) so the sys vars should be updated accordingly, right?

[bb7133]

lgtm and cc @morgo

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[bb7133]

/merge

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/merge

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[sticnarf]

LGTM

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• youjiali1995

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[sticnarf]

/merge

[ti-chi-bot]

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

Commit hash: 3d6e30c

[ti-srebot]

cherry pick to release-5.0 in PR #25118

[MyonKeminta]

So the point is, we changed the default value back to 'legacy'(pingcap/docs#5136) so the sys vars should be updated accordingly, right?

Currently, yes.