release-24.2.1-rc: sql: deflake TestConcurrentSchemaChanges

[blathers-crl]

Backport 1/1 commits from #129296 on behalf of @fqazi.

/cc @cockroachdb/release

fixes #129616

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Previously, TestConcurrentSchemaChanges was intermittently failing because the connection pool was not correctly being used. Some of operations like selecting if the declarative or legacy schema changer should be used were incorrectly being run on the main connection. To address this, this patch uses the dedicated connections per-thread.

Fixes: #129220
Fixes #129271
Fixes #129132

Release note: None

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Release justification: test only change

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
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  here).
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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is