Dockerfile: Bump golang version to 1.21

This bumps the Go version of the test container to 1.21.8, which is the
oldest supported Go version as of now.

Signed-off-by: Anoop C S <anoopcs@cryptolab.net>

testing/containers/ceph/Dockerfile

@@ -21,7 +21,7 @@ RUN true && \
     yum clean all && \
     true
 
-ARG GO_VERSION=1.19.5
+ARG GO_VERSION=1.21.8
 ENV GO_VERSION=${GO_VERSION}
 ARG GOARCH
 ENV GOARCH=${GOARCH}