build: update Dockerfile default go version to 1.21

[calvinrzachman]

I think there were some changes to how go.mod files are handled such that we need to move up to 1.21.

Fixes: https://github.com/lightninglabs/lndinit/actions/runs/9318276549/job/25650275261

/go/src/github.com/lightninglabs/lndinit &&  cd /go/src/github.com/lightninglabs/lndinit &&  git checkout v0.1.21-beta &&  make release-install:
6.002   git switch -
6.002 
6.002 Turn off this advice by setting config variable advice.detachedHead to false
6.002 
6.002 HEAD is now at 5f75c60 Merge pull request #41 from calvinrzachman/lnd-v0.18.0-beta
6.010 \033[0;32m Installing release lndinit.\033[0m
6.011 env CGO_ENABLED=0 go install -v -v -trimpath -ldflags=" -s -w -buildid= -X github.com/lightninglabs/lndinit.Commit=docker/v0.1.21-beta-lnd-v0.18.0-beta" -tags="" github.com/lightninglabs/lndinit
6.015 go: errors parsing go.mod:
6.015 /go/src/github.com/lightninglabs/lndinit/go.mod:199: invalid go version '1.21.10': must match format 1.23
6.015 make: *** [Makefile:90: release-install] Error 1

[guggero]

Should we go straight to 1.22.3?

[calvinrzachman]

Sounds good to me. Updated ✅