demo: Update Cloud SQL database to PostgreSQL 12
We are shipping and using PostgreSQL 12 on GitLab.com, so let's use this version to match production.
GitLab 14.0 uses PostgreSQL 12 (&3835 (closed)), and I believe we are using features like REINDEX CONCURENTLY
(https://paquier.xyz/postgresql-2/postgres-12-reindex-concurrently/) that are only available on PostgreSQL 12.
Edited by Stan Hu