Postgres 13 generally available in Google Cloud SQL (SDK 316.0.0)

Right now the link attached is outdated and only shows release notes until the version 315.0.0. However, after running manually `gcloud components update` you can see that version 316.0.0 (2020-10-27) mentions "Promoted Postgres 13 to GA" in the Cloud SQL section. Unfortunately I couldn't find any other link with more updated information.

That was very unexpected to me, because up until now I hadn't seen any mention of PG 13 even being in beta. That's great, since I was planning on upgrading a database to PG 12 this week, and now I can go straight to PG 13!