Update go version in go.mod

[lucacome]

Proposed changes

Problem: Go changed how the go directive works in Go 1.21. It now needs to be in the 1.21.x format.

go.dev/doc/toolchain#version
go.dev/ref/mod#go-mod-file-go

Solution: Update go.mod to add the patch version.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork