AArch64: change image rootfs from fedora to ubuntu

Ubuntu distribution is the most suitable one for AArch64 to build
image rootfs.
I think the size of rootfs is the key point we should consider most
and first.
And ubuntu has the smallest rootfs, only approximately 100MB.

Fixes: #2449

Signed-off-by: Penny Zheng <penny.zheng@arm.com>

versions.yaml

@@ -117,7 +117,7 @@ assets:
     url: "https://github.com/kata-containers/osbuilder"
     architecture:
       aarch64:
-        name: "fedora"
+        name: "ubuntu"
         version: "latest"
       ppc64le:
         name: "centos"