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

[Emanuel Borsboom]

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)