sql: PERFORM keyword is unsupported in plpgsql

**Describe the problem**

Trying to create a user defined function as follows:

```
CREATE FUNCTION "AddValues" (a INT, b INT) RETURNS INT
AS $$ SELECT $1 + $2 $$ LANGUAGE SQL;

CREATE FUNCTION "IsDate"(s TEXT) RETURNS BOOLEAN
AS $$
BEGIN
PERFORM s::DATE;
RETURN TRUE;
EXCEPTION WHEN OTHERS THEN
RETURN FALSE;
END;
$$ LANGUAGE PLPGSQL;
```

returns `plpgsql not supported in user-defined functions`

**Additional context**
What was the impact?

Add any other context about the problem here.

@fqazi This affects efcore.pg tests

Jira issue: CRDB-30455

Epic CRDB-49018