config, sysvar: fix the missing config of plugins.

[CbcWestwolf]

What problem does this PR solve?

Issue Number: close #34917

Problem Summary:

When the TiDB cluster upgrades to v6.1.0, the config about plugin is missed.

What is changed and how it works?

The bug is caused by the wrong convertion of [plugin] section in config.

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

Documentation

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

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• morgo
• zimulala

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/998c99efc2e20be88562bcf8c81eaa2b457f4ee7

[CbcWestwolf]

/cc bb7133 morgo

[zimulala]

LGTM

[zimulala]

/merge

[ti-chi-bot]

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

Commit hash: d44f209

[ti-srebot]

cherry pick to release-6.1 in PR #34933

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! New failing [2] after this pr merged.
These new failed integration tests seem to be caused by the current PR, please try to fix these new failed integration tests, thanks!

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-common-test	🟥 failed 2, success 9, total 11	17 min	New failing
idc-jenkins-ci-tidb/common-test	🟥 failed 2, success 10, total 12	7 min 11 sec	New failing
idc-jenkins-ci/integration-cdc-test	🟢 all 34 tests passed	26 min	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	6 min 48 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	6 min 20 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	5 min 54 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	5 min 29 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	4 min 4 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	3 min 46 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed