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