Clarify that currently we have to run on LTS 14
Clarify that currently we have to run on LTS 14

https://github.com/compiler-explorer/compiler-explorer/issues/3047#issuecomment-948032360
mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer