helm: upgrade to PostgreSQL 14

We are currently using PostgreSQL 12 in our Helm chart: https://github.com/reanahub/reana/blob/9cb65c8302290e699c4e23273b54bab1a4bcf376/helm/reana/templates/reana-db.yaml#L34

Given that we have already tested (and we are already using) PostgreSQL 14, we should update the internal database to the same version, so that local/dev environments use the same database version as REANA deployed at CERN.

Note that this represents a big issue for already-deployed instances of REANA that use the internal database, as a data migration is needed to move from PostgreSQL 12 to PostgreSQL 14. See the related docs PR https://github.com/reanahub/docs.reana.io/pull/170.

We should discuss what to do, as we have a few options:

Update to PostgreSQL 14 without making it possible to customise the version with Helm values, as we have done up until now. This is good to keep all deployments up to date and using the same version, but it means that REANA admins are forced to migrate their database when upgrading to a new version of REANA. This means we should also advertise in some way if a database migration is needed when releasing a new version of REANA.
Make the version of PostgreSQL customisable via Helm values, making PostgreSQL 14 as default. This means that admins can keep their old version of PostgreSQL without having to upgrade, but this also means that they might encounter incompatibilities if they keep the old version for too long. It might also happen that admins choose a newer version of PostgreSQL, which might also cause troubles.

reanahub / reana

helm: upgrade to PostgreSQL 14 #744