keyspace level gc: forbid update to ks level gc

[ystaticy]

What problem does this PR solve?

Issue Number: #8061

What is changed and how does it work?

Disables updating gc_management_type to keyspace_level_gc

Check List

Tests

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

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

Disables updating gc_management_type to keyspace_level_gc

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[ti-chi-bot]

Hi @ystaticy. Thanks for your PR.

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