RHEL-90: use C.UTF-8 for images that only have glibc-minimal-langpack

Change the default locale to `C.UTF-8` for RHEL-9.0.
For all the images which install `langpack-en`, keep using the
`en_US.UTF-8` locale. `C.UTF-8` is used as the default for:
 - edge-commit
 - edge-container
 - image-installer
 - qcow2
 - tar

Also change the default locale for the edge-simplified-installer
installer-tree pipeline, since its package set contains only
`glibc-minimal-langpack`.

Regenerate RHEL-90 and c9s image tests.

Fix #2206

Signed-off-by: Tomas Hozza <thozza@redhat.com>

test/data/manifests/centos_9-s390x-tar-boot.json

@@ -679,7 +679,7 @@
           {
             "type": "org.osbuild.locale",
             "options": {
-              "language": "en_US.UTF-8"
+              "language": "C.UTF-8"
             }
           },
           {
@@ -7570,7 +7570,7 @@
       "::1         localhost localhost.localdomain localhost6 localhost6.localdomain6"
     ],
     "locale": {
-      "LANG": "en_US.UTF-8"
+      "LANG": "C.UTF-8"
     },
     "machine-id": "",
     "os-release": {