chore: manual upgrade to Go 1.21

This attempts to fix multiple downstream golang upgrade problems by
manually upgrading to Go 1.21.

Separately, I'd like to:

1. Find out why the problem isn't being upgraded manually
2. Switch to binarySource=install as a Renovate configuration:
https://docs.renovatebot.com/self-hosted-configuration/#binarysource

.gitlab-ci-other-versions.yml

@@ -8,7 +8,7 @@ variables:
   # These are versions used in the Renovate runner image
   GL_COMMON_RENOVATE_GIT_VERSION: "2.41.0"  # datasource=github-tags depName=git/git
-  GL_COMMON_RENOVATE_GOLANG_VERSION: "1.20.2"  # datasource=golang-version depName=golang/go
+  GL_COMMON_RENOVATE_GOLANG_VERSION: "1.21.0"  # datasource=golang-version depName=golang/go
   GL_COMMON_RENOVATE_JB_VERSION: "0.5.1"  # datasource=github-releases depName=jsonnet-bundler/jsonnet-bundler
   GL_COMMON_RENOVATE_JSONNET_TOOL_VERSION: "1.9.1"  # datasource=gitlab-releases depName=gitlab-com/gl-infra/jsonnet-tool
   GL_COMMON_RENOVATE_NODEJS_VERSION: "18.17.1"  # datasource=node depName=nodejs/node