CI: eliminate gcc6 and clang6 builds

Currently our linux builds include:

• gcc6
• gcc10
• clang6
• clang12

The minimum versions that support C++17 are gcc8 and clang5. I propose we eliminate gcc6 and clang6 to ensure we have mature cpp17 support, stop fussing over ancient compiler bugs, and save a few CI minutes.

If we want to test something newer, I think gcc11 and clang16 are good choices. Those are the versions that started defaulting to cpp17.