Update "Implementations" section of VERSION-RANGE-SPEC.md

[johnmhoran]

While working on converting VERSION-RANGE-SPEC.rst to .md I've come to the "Implementations" section, immediately above which is the following todo, which appears to refer to the "Implementations" section:

TODO: add Rust, composer and archlinux, nginx, tomcat, apache.

I'm removing that todo from the .md version, which we can update as needed when the added content is ready.

[tschmidtb51]

Maybe, you want to add the Rust and Javascript one at some point: https://github.com/csaf-rs/vers-rs (still in early stages though).