schematracker(dm): Increase max-index-length config for dm schema tracker (#11465)

[ti-chi-bot]

This is an automated cherry-pick of #11465

What problem does this PR solve?

Issue Number: close #11459

What is changed and how it works?

DM schema tracker uses the default tidb max-index-length config (3072), regardless of the config value of the downstream tidb. If the downstream tidb configures a larger value, then ddls that execute successfully against the downstream TiDB could fail in the schema tracker.

While DM's schema tracker makes an effort to suppress these kinds of errors, it only applies to single column, non-unique indexes.

This change updates the config of DM schema tracker to use the maximum max-index-length, or 3072*4 (12288). Ideally, DM would use the config of the downstream TiDB directly. However, we already are less strict w/ queries against DM schema tracker (running queries in non-strict, suppressing errors), and can extend the max length to maximum allowable. This way, we can ensure that any DDLs that would execute successfully against the downstream TiDB would be successful in the schema tracker, while not necessarily guaranteeing that any DDLs that fail against the downstream TiDB necessarily fail against DM's schema tracker.

Check List

Tests

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

Questions

Will it cause performance regression or break compatibility?

N

Do you need to update user documentation, design documentation or monitoring documentation?

N

Release note

Fix issue where DDLs could fail in DM schema tracker due to exceeding the default max-index-length

[D3Hunter]

@lance6716

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: D3Hunter, lance6716

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