Upgrade Checkmate's Postgres to a newer version before May 2024

[seanh]

Checkmate currently uses Postgres 11 which is supported by the Postgres project until Nov 9 2023 and supported by AWS RDS until Nov 2023. We need to upgrade to a new version of Postgres before support runs out.

[seanh]

Update: RDS added Postgres 11.20 which they're supporting until May 2024 which buys us a few more months before we have to upgrade past 11

[seanh]

According to https://www.postgresql.org/support/versioning/ there will be a final Postgres 11.* release on November 9th 2023. Presumably RDS will then support that final 11.* version until some time beyond May 2024 (which is when they're supporting the currently-released Postgres 11.20 until). I don't think we know for sure that RDS will even support this final version of Postgres 11 (?) and we don't know exactly how long they'll support it for until they publish that. But we may get an extension beyond May 2024