sql: Mistakenly disallow UNIQUE WITHOUT INDEX NOT VALID in CREATE TABLE stmt

[Xiang-Gu]

We can add a not valid unique-without-index constraint to a table via ADD UNIQUE WITHOUT INDEX (cols) NOT VALID, but we disallow creating a table with a not valid unique-without-index constraint.

This is because commit 8dd20ad disallowed add not valid primary and unique constraint (with the intention of index-backed-unique constraint). Unique-without-index is not backed by any index and we should allow it. Just like any other not valid constraint (e.g. check, FK) in create table in that same commit, the NOT VALID part will be dropped and is treated as a normal unique-without-index.

Jira issue: CRDB-33992

Epic CRDB-30346

[Xiang-Gu]

I don't think we want to backport the fix to 23.2 so I'm tagging it as a docs-known-limitation issue.