Bump centos from 7 to 8 in /docker/vpp-centos-userspace-cni

Bumps centos from 7 to 8.

---
updated-dependencies:
- dependency-name: centos
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Diff for: docker/vpp-centos-userspace-cni/Dockerfile

@@ -5,7 +5,7 @@
 
 
 # -------- Builder stage.
-FROM centos:7@sha256:be65f488b7764ad3638f236b7b515b3678369a5124c47b8d32916d6487418ea4
+FROM centos:8@sha256:a27fd8080b517143cbbbab9dfb7c8571c40d67d534bbdee55bd6c473f432b177
 
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]