release-24.3: sql: relax validation for IDENTITY columns

[blathers-crl]

Backport 1/1 commits from #139084 on behalf of @rafiss.

/cc @cockroachdb/release

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The validations added in 4f99f9c were
too strict. This patch makes it so we can continue having IDENTITY
columns that are not backed by a sequence. This can happen if a SERIAL
column is created with IDENTITY, and serial_normalization=rowid.

informs #138925
informs #138972

Release note (bug fix): Fixed a bug where columns created with GENERATED ... BY IDENTITY with the SERIAL type could incorrectly fail internal validations.

---

Release justification: high priority bug fix; fixes overly strict validation that could break functionality

[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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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

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[cockroach-teamcity]

This change is