multi: update to go 1.21

[jamaljsr]

I needed to update to go v1.21 to build tapd. Now when building litd, it results in the error:

 Installing lightning-terminal.
go install -v -trimpath -tags="litd autopilotrpc signrpc walletrpc chainrpc invoicesrpc watchtowerrpc neutrinorpc peersrpc" -ldflags " -X github.com/lightningnetwork/lnd/build.Commit=lightning-terminal-v0.12.0-alpha-6-gd782cbbfef73145c992e034e3641cef9bcc0f135 -X github.com/lightningnetwork/lnd/build.CommitHash=d782cbbfef73145c992e034e3641cef9bcc0f135 -X github.com/lightningnetwork/lnd/build.GoVersion= -X github.com/lightningnetwork/lnd/build.RawTags=litd,autopilotrpc,signrpc,walletrpc,chainrpc,invoicesrpc,watchtowerrpc,neutrinorpc,peersrpc -X github.com/lightninglabs/lightning-terminal.appFilesPrefix= -X github.com/lightninglabs/lightning-terminal.Commit=v0.12.0-alpha-6-gd782cbbfef73145c992e034e3641cef9bcc0f135 -X github.com/lightninglabs/loop.Commit=v0.26.4-beta -X github.com/lightninglabs/pool.Commit=v0.6.4-beta.0.20231003174306-80d8854a0c4b -X github.com/lightninglabs/taproot-assets.Commit=v0.3.0" github.com/lightninglabs/lightning-terminal/cmd/litd
go: updates to go.mod needed; to update it:
	go mod tidy
make: *** [Makefile:131: go-install] Error 1

When running go mod tidy, it produced the diff here in this PR. I also updated the Docker and GH workflow files to use go v1.21 as well.

I have tested this branch while using go v1.19 and it still works, so this is a backward compatible change.

[bitromortac]

LGTM 🎉 (Edit: just came to my mind whether we want to wait until all subdaemons have been upgraded to 1.21, to let their unit/integration tests pass)

I used go v1.21.3 to run go mod tidy and it added a line toolchain go1.21.3 which enforces a minimal version of go (see https://go.dev/blog/toolchain). However, starting from a mod file that already contains go 1.21 but not the toolchain line, the toolchain line is not added with go mod tidy, so it is optional. Since the taproot assets repo also doesn't have it, I think it's fine.

[jamaljsr]

just came to my mind whether we want to wait until all subdaemons have been upgraded to 1.21, to let their unit/integration tests pass

I didn't mention this initially but the immediate need for this fix is because the cron job that updates the API Docs site daily has been failing for over a week now due to this issue. Unless there's a compatibility issue with the other sub-daemons, it would be better to not wait.

[bitromortac]

Just looked at the lnd unit/itests, they run with go 1.21, so I think it should be fine

[ViktorTigerstrom]

LGTM after rebase!

[jamaljsr]

Rebased. Merging now.