release-24.1: pgwire: change sql.defaults.results_buffer.size default to 512KiB

[blathers-crl]

Backport 1/1 commits from #124580 on behalf of @rafiss.

/cc @cockroachdb/release

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When this setting was added in PR #32366, the default value did not receive much scrutiny. Using a bigger value is beneficial since it will reduce the chance for clients using READ COMMITTED transactions to hit errors that cannot automatically be retried within CockroachDB. (The reason being that if results were already sent back to the client since the buffer filled up, it's not safe to retry that operation.)

512KiB is still pretty small, but should prevent more errors than the old default of 16KiB.

fixes #124049
Release note (sql change): The default value of the sql.defaults.results_buffer.size cluster setting was changed from 16KiB to 512KiB.

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Release justification: low risk high benefit config 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.
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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 

[fqazi]

Reviewed 9 of 9 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @kyle-a-wong and @nvanbenschoten)