* build: update to go1.21
* go: eliminate helpers in favor of min/max
* build: run go mod tidy
* build: swap depguard for semgrep
* command: fixup broken tls error check on go1.21
Go released a security update to fix build-time code injection and execution via
CGO. This doesn't impact already-released versions of Nomad, just the build
toolchain, so we won't be releasing a Nomad security update to go with it.
