Graduate "Forensic Container Checkpointing" to Beta

As defined in the existing KEP the steps to graduate from Alpha to Beta
are

   At least one container engine has to have implemented the
   corresponding CRI APIs to introduce e2e test for checkpointing.

   - [ ] Enable the feature per default
   - [ ] No major bugs reported in the previous cycle

CRI-O implemented the corresponding CRI RPC and no major bugs
have been reported since the initial release in 1.25.

Signed-off-by: Adrian Reber <areber@redhat.com>

keps/prod-readiness/sig-node/2008.yaml

@@ -1,3 +1,5 @@
 kep-number: 2008
 alpha:
   approver: "@ehashman"
+beta:
+  approver: "@deads2k"