ci: Move from 'centos-8' to 'almalinux-8'

Upstream lcitool suggests that as a solution to 'centos-8' being
removed.

Move also the website and other jobs to depend on
'x86_64-almalinux-8-container'.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

ci/manifest.yml

@@ -6,7 +6,7 @@ gitlab:
   project: libvirt
 
 targets:
-  centos-8:
+  almalinux-8:
     jobs:
       - arch: x86_64
         variables: