Update mattermost dev DB to use postgresql 15

General MR

Summary

Urgent upgrade of PostgreSQL in the mattermost dev subchart to a supported version

Relevant logs/screenshots

$ kubectl get pods -n mattermost
NAME                          READY   STATUS    RESTARTS   AGE
mattermost-postgresql-0       2/2     Running   0          3m47s
mattermost-minio-pool-0-0     3/3     Running   0          3m35s
mattermost-84dc677967-szcst   2/2     Running   0          3m46s

$ kubectl events -n mattermost | grep postgresql
3m42s                   Normal    Pulled                            Pod/mattermost-postgresql-0                             Successfully pulled image "registry1.dso.mil/ironbank/opensource/postgres/postgresql:15.7" in 8.609s (8.609s including waiting)
3m42s                   Normal    Created                           Pod/mattermost-postgresql-0                             Created container postgresql
3m42s                   Normal    Started                           Pod/mattermost-postgresql-0                             Started container postgresql

Linked Issue

Closes #155

Upgrade Notices

BREAKING CHANGE If using the sub-chart Postgres database for Mattermost, you will be required to perform a pg_dump before upgrading and then restore the database and data to the new pod in order to keep data. There is no clean upgrade path from prior PostgreSQL versions to the new version. This sub-chart in-cluster database is NOT a production setup/recommended installation.

BREAKING CHANGE If you are setting a custom username, password or database for the sub-chart Postgres database for mattermost, the credential settings have moved.

Old setting	New setting
postgresql.postgresqlUsername	postgresql.auth.username
postgresql.postgresqlPassword	postgresql.auth.password
postgresql.postgresqlDatabase	postgresql.auth.database