sql: use unimplemented error rather than syntax error for PL/pgSQL

[rafiss]

fixes #96720

This change does not affect anything user-facing, but it allows us to parse CREATE FUNCTION statements that use LANGUAGE plpgsql. The plpgsql grammar is still not supported, so this will still show an error to the user. However, since parsing succeeds, the statement will be sent to telemetry logs, allowing us to analyze which parts of the plpgsql grammar that customers are trying to use.

Release note: None

[cockroach-teamcity]

This change is 

[mgartner]

Thanks for doing this!

Reviewed 9 of 9 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @chengxiong-ruan)

[chengxiong-ruan]

nice~

[rafiss]

tftr!

bors r=chengxiong-ruan,mgartner

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 6b34dfb to blathers/backport-release-22.2-96726: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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