chore: manual upgrade to Go 1.21
This attempts to fix multiple downstream golang upgrade problems by manually upgrading to Go 1.21.
Separate from this MR, I'd like to:
- Find out why the problem isn't being upgraded manually
- Switch to binarySource=install as a Renovate configuration: https://docs.renovatebot.com/self-hosted-configuration/#binarysource
Edited by Andrew Newdigate