feat: upgrade to go 1.21

[crandles]

Update all packages to 1.21, and update tests to cover 1.21 -> 1.23.

I was going to start with 1.23 -- I can update if preferred. At least with 1.21, you can build the later toolchains, so you can start to update individual packages as needed.

Additional changes:

• go mod tidy of all packages
• go work sync

---

There are many packages in the repository -- I'm not sure what your build tool of choice is, but here are some non-fancy makefile targets I've been using:

update-deps:
	GO_ARGS="mod tidy" $(MAKE) go
	GO_ARGS="work sync" $(MAKE) go
	GO_ARGS="test -v" $(MAKE) go
	GO_ARGS="build -v" $(MAKE) go

GO_ARGS ?= -h
go:
	find . -type d -not -path '*/.*' -exec sh -c 'cd {} && go $(GO_ARGS)' \;