feat!: remove the deprecated extraVolumeMounts value

`extraVolumeMounts` was deprecated in 6.0.0 in favour of separate mounts for the init phase and the runtime container,
but it was never removed. It had turned into a trap: both helpers treated it as a fallback that silently disappeared as
soon as `extraInitVolumeMounts` respectively `deployment.gitea.volumeMounts` contained a single entry, so adding one
new mount could unmount everything configured through the old value. The documented successor
`extraContainerVolumeMounts` had meanwhile been removed itself, which left the deprecation note pointing at a value
that already fails the deprecation check.

Removing it makes the mount configuration explicit: `extraInitVolumeMounts` for the init containers,
`deployment.gitea.volumeMounts` for the Gitea container, and both when a volume is needed in both places.

The GPG init container used `extraVolumeMounts` directly instead of the shared helper and therefore ignored
`extraInitVolumeMounts`, unlike the three other init containers. It now uses `gitea.init-additional-mounts` as well.

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

values.yaml

@@ -926,13 +926,6 @@ extraContainers: []
 
 ## @param extraInitVolumeMounts Mounts that are only mapped into the init-containers. Can be used for additional preconfiguration.
 extraInitVolumeMounts: []
-
-## @deprecated The extraVolumeMounts variable has been split two:
-## - deployment.gitea.volumeMounts
-## - extraInitVolumeMounts
-## As an example, can be used to mount a client cert when connecting to an external Postgres server.
-## @param extraVolumeMounts **DEPRECATED** Additional volume mounts for init containers and the Gitea main container.
-extraVolumeMounts: []
 # - name: postgres-ssl-vol
 #   readOnly: true
 #   mountPath: "/pg-ssl"