vector: array_agg can exceed maximum number of dimensions

Found when running the regress suite from `pgvector` (`cast.sql` file).

The following query results in error in PG but succeed in CRDB:
```sql
-- PG returns '54000: vector cannot have more than 16000 dimensions'
SELECT array_agg(n)::vector FROM generate_series(1, 16001) n;
```
We hard-coded the same maximum number of dimensions, so this could perhaps lead to undefined behavior down the line.

Jira issue: CRDB-50614