[Haskell-cafe] ANN: second release candidate for stack-2.7

You can download binaries for this pre-release from https://github.com/commercialhaskell/stack/releases/tag/rc%2Fv2.7.0.3 Please test it and let us know if you run into any trouble. If all goes well, we expect to release the final version in X weeks. Release notes: * Stack's bindists are now built with GHC 8.10.4, and our `stack.yaml` has moved to lts-17.10. This means Stack can now be built on macOS 11.0 (Big Sur) without ugly workarounds. GHC 8.8 is now the minimum GHC version supported for building Stack itself (but this does not effect _users_ of Stack, which still supports using much older GHC versions). **Changes since v2.7.0.1:** (no changes)