update to go1.21.6, go1.20.13

go1.21.6 (released 2024-01-09) includes fixes to the compiler, the runtime, and
the crypto/tls, maps, and runtime/pprof packages. See the Go 1.21.6 milestone on
our issue tracker for details:

- https://github.com/golang/go/issues?q=milestone%3AGo1.21.6+label%3ACherryPickApproved
- full diff: https://github.com/golang/go/compare/go1.21.5...go1.21.6

go1.20.13 (released 2024-01-09) includes fixes to the runtime and the crypto/tls
package. See the Go 1.20.13 milestone on our issue tracker for details:

- https://github.com/golang/go/issues?q=milestone%3AGo1.20.13+label%3ACherryPickApproved
- full diff: https://github.com/golang/go/compare/go1.20.12...go1.20.13

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

contrib/Dockerfile.test

@@ -29,7 +29,7 @@
 #   docker run --privileged containerd-test
 # ------------------------------------------------------------------------------
 
-ARG GOLANG_VERSION=1.21.5
+ARG GOLANG_VERSION=1.21.6
 ARG GOLANG_IMAGE=golang
 
 FROM ${GOLANG_IMAGE}:${GOLANG_VERSION} AS golang