chore: update min version to 1.21 (#4392)

Noticed this was still needed in #4383. Note we should also switch
latest to 1.23, but I did not in this PR because the image has not
been built yet.

Co-authored-by: Eric Schmidt <erschmid@google.com>
Co-authored-by: Marc Dougherty <muncus@users.noreply.github.com>

testing/kokoro/earliest-version.cfg

@@ -3,5 +3,5 @@
 # Configure the docker image for kokoro-trampoline.
 env_vars: {
     key: "TRAMPOLINE_IMAGE"
-    value: "gcr.io/golang-samples-tests/go120"
+    value: "gcr.io/golang-samples-tests/go121"
 }