ubuntu-latest and macos-latest are evergreen, so don't need to be manually updated anymore every year or two
because requires "nim >= 1.6", it's safe to only use -mm:foo, even on Nim 1.6, rather than continue to include the fallback to --gc:foo for 1.2 and 1.4
Regarding the other changes:
ubuntu-latestandmacos-latestare evergreen, so don't need to be manually updated anymore every year or tworequires "nim >= 1.6", it's safe to only use-mm:foo, even on Nim 1.6, rather than continue to include the fallback to--gc:foofor 1.2 and 1.4