ddl: set tiflash placement group index to 120 (#37179)

[ti-chi-bot]

This is an automated cherry-pick of #37179

What problem does this PR solve?

Issue Number: ref #37171

What is changed and how it works?

1. Set the group_index of placement rule group 'tiflash' to 120 when adding a tiflash placement rule.
2. Fix the group_index in PollTiFlashRoutine when the background task started for old cluster.

Check List

Tests

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

Manual test1:

1. Create a table with tiflash replica in the old cluster.
2. Upgrade one tidb instance of the cluster to the code of this PR, the tiflash group_index was fixed to 120.

Manual test2:

1. Setup a new cluster with the code of this PR.
2. Create a new table with tiflash replica, the tiflash group_index is 120.

Release note

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• bb7133
• xhebox

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[bb7133]

LGTM

[lcwangchao]

/merge

[ti-chi-bot]

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

Commit hash: 399bab2

[xhebox]

/merge