config: remove per-table-memory-quota min value check (#3022)

[ti-chi-bot]

This is an automated cherry-pick of #3022

What problem does this PR solve?

Remove per-table-memory-quota min value check, because it is
possible to capture thousands of tables, and in this case, 6MB is
too large.

Also, it changes the default quota from 20MB to 10MB. Tests show
that no obvious bottleneck for 10MB.

Cc https://github.com/pingcap/ticdc/pull/2561

Metrics: https://gist.github.com/overvenus/183013d6b68c73f6b31d0dc1e5fb2bec

Quota = 1MB vs Quota = 10MB vs Quota = 20MB
4.5s vs 49s vs 58s
41k vs 361k vs 474k

Check List

Tests

• Manual test (add detailed scripts or steps below)

See above.

Related changes

• Need to cherry-pick to the release branch

Release note

None

[ti-chi-bot]

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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 503ae4a

[ti-chi-bot]

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