doc builds fail with Go 1.23

[cyphar]

We need to switch to go.mod I believe, as go run .tool/doc-version.go doesn't work anymore without a go.mod:

% make docs
go run ./.tool/version-doc.go > version.md
.tool/version-doc.go:11:2: no required module provides package github.com/opencontainers/runtime-spec/specs-go: go.mod file not found in current directory or any parent directory;
see 'go help modules'
make: *** [Makefile:55: version.md] エラー 1
make docs  0.05s user 0.08s system 131% cpu 0.096 total

[tianon]

In my testing, module github.com/opencontainers/runtime-spec as the only contents of go.mod is sufficient, so we could alternatively create/ignore that file in the Makefile 🤔

The annoying bit about go.mod here is that our tags/versions have nothing to do with the "stability" or API of our Go code -- they're the version of the spec. So another alternative could be to add go.mod inside specs-go specifically to avoid that, but then we'd need to adjust .tool appropriately too.

[tianon]

Alternatively, we could replace .tool/version-doc.go with a simple awk, sed, jq, etc across specs-go/version.go 🤷 (it's not exactly a complex file, and what we're extracting from it isn't exactly complex either)

Current successfully generated content:

**Specification Version:** *1.2.1*