feat(deployment)!: move security contexts into the deployment dict

`podSecurityContext` and `containerSecurityContext` are both Deployment-scoped: the former is rendered into
`spec.template.spec.securityContext`, the latter into the securityContext of the Gitea container and the
chart-managed init containers. Keeping them at the top level hid that pod/container distinction behind a
naming convention and separated them from the other pod- and container-scoped settings that already live
under `deployment` and `deployment.gitea`.

`podSecurityContext` therefore becomes `deployment.securityContext` and `containerSecurityContext` becomes
`deployment.gitea.securityContext`, which makes the scope obvious from the values path alone and continues
the consolidation started with `deployment.gitea.env`, `deployment.gitea.resources` and
`deployment.gitea.image`.

The template helpers keep their argument-based signatures, because `gitea.containerSecurityContext` is also
used by the Helm test pod and is not bound to a single values path.

Both removed keys are covered by the deprecation check so that a silently dropped security context cannot
lead to containers unexpectedly running as root or without the configured capability set.

BREAKING CHANGE: `podSecurityContext` and `containerSecurityContext` no longer exist. Use
`deployment.securityContext` and `deployment.gitea.securityContext` instead. Installations that still set
the old keys will fail to render unless `checkDeprecation` is set to `false`.

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

unittests/helm/deployment/openshift.yaml

@@ -80,11 +80,13 @@ tests:
       openshift:
         enabled: true
         hostUsers: true
-      podSecurityContext:
-        fsGroup: 1000620000
-      containerSecurityContext:
-        runAsUser: 1000620000
-        runAsGroup: 1000620000
+      deployment:
+        securityContext:
+          fsGroup: 1000620000
+        gitea:
+          securityContext:
+            runAsUser: 1000620000
+            runAsGroup: 1000620000
     asserts:
       - equal:
           path: spec.template.spec.hostUsers