Drop support for Go older than 1.18

[SuperQ]

We don't support Go older than 1.21 in the Go modules file, so we can drop the special handling for older Go in the version package.

[ArthurSens]

Sorry if the comment sounds lazy, I'm still stuck between airports and not very familiar with runtime libraries.

[ArthurSens]

I'm reading the BuildSettings docs and I can't find the -tags keys.

Is this something that was removed in more recent versions of go? Or just missing in the docs?

// A BuildSetting is a key-value pair describing one setting that influenced a build.
//
// Defined keys include:
//
//   - -buildmode: the buildmode flag used (typically "exe")
//   - -compiler: the compiler toolchain flag used (typically "gc")
//   - CGO_ENABLED: the effective CGO_ENABLED environment variable
//   - CGO_CFLAGS: the effective CGO_CFLAGS environment variable
//   - CGO_CPPFLAGS: the effective CGO_CPPFLAGS environment variable
//   - CGO_CXXFLAGS:  the effective CGO_CXXFLAGS environment variable
//   - CGO_LDFLAGS: the effective CGO_LDFLAGS environment variable
//   - GOARCH: the architecture target
//   - GOAMD64/GOARM/GO386/etc: the architecture feature level for GOARCH
//   - GOOS: the operating system target
//   - vcs: the version control system for the source tree where the build ran
//   - vcs.revision: the revision identifier for the current commit or checkout
//   - vcs.time: the modification time associated with vcs.revision, in RFC3339 format
//   - vcs.modified: true or false indicating whether the source tree had local modifications
type BuildSetting struct {
	// Key and Value describe the build setting.
	// Key must not contain an equals sign, space, tab, or newline.
	// Value must not contain newlines ('\n').
	Key, Value string
}

[SuperQ]

Hmm, good question. This isn't new code, just moving it from info_go118.go. @bboreham added it in #455.

[bboreham]

The list is not claimed to be exclusive; it says it “includes” those things.