Update build instructions in the documentation

[TheSunCat]

Hi! I was having issues on the latest release with singing in (just boots me back to the "Sign in with Github" screen), so I decided to try the latest version from git before looking into writing an issue about this. However, the branch does not appear to build on my machine. I followed the instructions at the end of the README, but one of the dependencies appears to have changed URL from https://github.com/NamanShergill/gql to https://github.com/gql-dart/gql. I was able to get flutter pub get to run without errors by also adding a line to force it to use a very old commit:

      ref: c4fc5c50e6c772b4264d2dd173c4bac1a940c8cf

However, subsequently running flutter build apk yielded a message about Java and Gradle incompatibilities, linking me here. I followed the instructions to update the Gradle version, and now I'm getting a bunch of build errors, so I think I'm definitely missing something:

build_logs.txt