⚠️ Upgrade Postgres version from 14.6 to 16.2

[KevinJBoyer]

Ticket

Resolves #523

Changes

• Bump from 14.6 to 16.2
• Add documentation to describe how to do a major version change if desired.

Small tweaks to the code to make upgrading major versions slightly easier:

• Specify allow_major_version_change = false -- this is the default, but making it explicit makes it easier for teams to know where this should be updated when they need to change it to true
• Remove revocation of create on public schema from role_manager.py -- this is now the default as of Postgres 15, but I'm leaving this behavior "as is" so folks can continue to use the infra template with past versions of Postgres safely.
• Suffix the name of the parameter group with the major engine version. This saves a small manual step when doing a major database change, since otherwise users have to edit the Terraform to assign a new name.

Context for reviewers

• 16.2 was released in April 2024 and includes bug fixes for 16.1.
• You can see a full list of changes from 14.6 -> 16.2 here: https://why-upgrade.depesz.com/show?from=14.6&to=16.2&keywords=
• Note that the idea behind providing documentation is that a team should be able to upgrade to a future release of the infra template without being required to also update their database at the same time, so teams in such a position can upgrade the infra template and then manually revert the DB version number to their desired state and then upgrade in the future at their leisure.

Testing

• I manually tested the instructions on my AWS account:
  

• I reviewed this list for potential incompatibilities between 14 and 15, and this list of potential incompatibilities between 15 and 16.

• Besides a change to default permissions, I don't think there are any incompatibilities with the infra template.

  • One exception, I'm a bit confused about this release note in 16: Role inheritance now controls the default inheritance status of member roles added during [GRANT](https://www.postgresql.org/docs/16/sql-grant.html)

• I will put up sibling PRs that check this for the Application Flask template (and the NextJS one if it uses a database as well.)

[KevinJBoyer]

Once committed, this documentation will be the second reference ever to this error on Google 🙃

[lorenyu]

Rollout notes: Once this is merged we'll need to run through these migration steps on the platform-test and https://github.com/navapbc/platform-test-flask repos. Lemme know if you need access to the AWS accounts for those