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>

Chart.lock

@@ -1,9 +1,6 @@
 dependencies:
-- name: postgresql
-  repository: oci://registry-1.docker.io/bitnamicharts
-  version: 16.7.27
 - name: valkey
   repository: https://valkey.io/valkey-helm
   version: 0.10.0
-digest: sha256:40619b69b9527f2044de588b227cf6b174011d4aa16cc048690cc970ce9ecd35
-generated: "2026-10-03T23:18:30.630364276+02:00"
+digest: sha256:c3deb43d02228f143e1c9525f5ce06dc6a78646e763c7a73c5ec8ea0101ed2f5
+generated: "2026-10-04T00:24:11.331968211+02:00"