Update go version to 1.21 to match k/k

prow.sh

@@ -86,7 +86,7 @@ configvar CSI_PROW_BUILD_PLATFORMS "linux amd64 amd64; linux ppc64le ppc64le -pp
 # which is disabled with GOFLAGS=-mod=vendor).
 configvar GOFLAGS_VENDOR "$( [ -d vendor ] && echo '-mod=vendor' )" "Go flags for using the vendor directory"
 
-configvar CSI_PROW_GO_VERSION_BUILD "1.20" "Go version for building the component" # depends on component's source code
+configvar CSI_PROW_GO_VERSION_BUILD "1.21" "Go version for building the component" # depends on component's source code
 configvar CSI_PROW_GO_VERSION_E2E "" "override Go version for building the Kubernetes E2E test suite" # normally doesn't need to be set, see install_e2e
 configvar CSI_PROW_GO_VERSION_SANITY "${CSI_PROW_GO_VERSION_BUILD}" "Go version for building the csi-sanity test suite" # depends on CSI_PROW_SANITY settings below
 configvar CSI_PROW_GO_VERSION_KIND "${CSI_PROW_GO_VERSION_BUILD}" "Go version for building 'kind'" # depends on CSI_PROW_KIND_VERSION below