Add "safe version" in Conformance Overview

[cdervis]

Sometimes you want to know which version of a toolchain is required to safely use a certain language standard.

For example: "Which version of GCC allows me to safely use the majority of C++20?"

Currently, users have to find this out by using the respective toolchain filters and by inspecting the matching results.

We want to give users a good overview for each toolchain in the Conformance Overview. The threshold for what counts as "the majority" of a language's conformance is still to be determined, but 80% might be a good start.