release-24.1: sql: Skip constraints in pg_constraint that are being dropped.

[spilchen]

Backport 1/1 commits from #137860.

/cc @cockroachdb/release

---

When querying the pg_catalog.pg_constraint table during the process of dropping a constraint, the query could fail under certain conditions. For example, if the declarative schema changer (DSC) is in the middle of dropping a check constraint, the query could produce the following error:

ERROR: column "crdb_internal_column_2_name_placeholder" does not exist, referenced in "crdb_internal_column_2_name_placeholder IS NOT NULL" (SQLSTATE 42703)

This issue arises because, when the DSC is also dropping the column associated with the constraint, it removes the column's name from the descriptor. Consequently, any expressions referencing the column are updated with a placeholder name. While the DSC ensures that the column is no longer visible, the pg_constraint virtual table inspects the descriptor directly, encountering the placeholder name and attempting to resolve it. This fix ensures that constraints in the process of being dropped are skipped in pg_constraint, preventing such errors.

This shows up in the schema changer workload because it uses pg_constraint when building out the random op to run.

Epic: none
Closes #137513
Release note (bug fix): Fixed a bug where querying the pg_catalog.pg_constraint table while the schema changer was dropping a constraint could result in a query error.
Release justification: low-risk bug fix

[blathers-crl]

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[cockroach-teamcity]

This change is 

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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