feat!: remove the postgresql sub-chart

With `cloudnativePG` in place, the bundled Bitnami `postgresql` sub-chart is the second in-chart way to run a
database, and the worse one. It is a single, non-HA instance without failover, backup or managed major-version
upgrades, and it ships images that Bitnami no longer distributes freely. Keeping it around means maintaining two
code paths for the same job, including the mutual-exclusion assertion between both dependencies and a second set
of DNS, credential and image helpers.

Removing it leaves exactly two supported choices: a CloudNativePG `Cluster` rendered by this chart, or an
external database configured via `gitea.config.database`. Both scale from a single instance to a HA setup -
`cloudnativePG.instances=1` covers the use case the `postgresql` sub-chart served - so nothing is lost besides
the duplication.

BREAKING CHANGE: The `postgresql` sub-chart and all `postgresql.*` values have been removed. There is no
in-place migration path: dump the database of the old `postgresql` release with `pg_dump`, remove the
`postgresql` values, deploy the chart with `cloudnativePG.enabled=true` and restore the dump. Alternatively
point `gitea.config.database` at an externally managed database. Setting `postgresql` now fails the release with
a pointer to the changelog.

Co-authored-by: Copilot <copilot@github.com>

unittests/helm/config/database.yaml

@@ -46,14 +46,3 @@ tests:
             HOST=gitea-unittests-postgresql-rw.testing.svc.cluster.local:5432
             NAME=gitea-database
             USER=gitea-username
-
-  - it: "Fail when postgresql and cloudnativePG are enabled at the same time"
-    template: templates/secret_config.yaml
-    set:
-      postgresql:
-        enabled: true
-      cloudnativePG:
-        enabled: true
-    asserts:
-      - failedTemplate:
-          errorMessage: "Only one of postgresql or cloudnativePG can be enabled at the same time."