RHEL-90: use XBOOTLDR partition GUID for /boot

Use the Extended Bootloader Partition GUID for `/boot`, instead of the
Linux filesystem data GUID. This is useful for autodetection of a
partition purpose based on its GUID without reading the `/etc/fstab`
first.

Ensure that when creating mountpoints, e.g. when converting the
partitions layout to LVM, the `/boot` partition get the proper GUID
assigned.

Regenerate RHEL-90 and centos-9 image test cases.

Related to https://bugzilla.redhat.com/show_bug.cgi?id=2057231

test/data/manifests/centos_9-aarch64-openstack-boot.json

@@ -936,7 +936,7 @@
                 {
                   "size": 1024000,
                   "start": 411648,
-                  "type": "0FC63DAF-8483-4772-8E79-3D69D8477DE4",
+                  "type": "BC13C2FF-59E6-4262-A352-B275FD6F7172",
                   "uuid": "CB07C243-BC44-4717-853E-28852021225B"
                 },
                 {
@@ -10350,7 +10350,7 @@
         "partuuid": "CB07C243-BC44-4717-853E-28852021225B",
         "size": 524288000,
         "start": 210763776,
-        "type": "0FC63DAF-8483-4772-8E79-3D69D8477DE4",
+        "type": "BC13C2FF-59E6-4262-A352-B275FD6F7172",
         "uuid": "0194fdc2-fa2f-4cc0-81d3-ff12045b73c8"
       }
     ],