misc: Move to golang 1.21

Unfortunately, this also breaks go-mod-upgrade with:

upgrade failed error=Error running go command to discover modules: exit
status 1 stderr=go: loading module retractions for
golang.org/x/mod@v0.16.0: version "v0.17.0" invalid: resolves to version
v0.17.1-0.20240315155916-aa51b25a4485 (v0.17.0 is not a tag) go: loading
module retractions for golang.org/x/sync@v0.6.0: version "v0.7.0"
invalid: resolves to version v0.7.1-0.20240304172602-14be23e5b48b
(v0.7.0 is not a tag)

.github/workflows/test.yaml

@@ -27,9 +27,9 @@ jobs:
           # macos tests are currently failing in CI
           #- macos-latest
         golang_version:
-          # TODO: add 1.21.x and tip
+          # TODO: add 1.22.x and tip
           # minimum required and latest published go_version
-          - "1.20"
+          - "1.21"
         test_block:
           - basic
           - shell