Cleanup a few repo docs (#1987)

The `MAINTENANCE.md` file in particular hasn't been updated in 3 years.

Author: parlough

CONTRIBUTING.md

@@ -71,7 +71,7 @@ Each app should:
   file used throughout the repo. This file include a base set of analyzer
   conventions and lints.
 * Have no analyzer errors or warnings.
-* Be formatted with `flutter format`.
+* Be formatted with `dart format`.
 * Include at least one working test in its `test` folder.
 * Be wired into the list of projects in the CI scripts for [stable](tool/flutter_ci_script_stable.sh), 
   [beta](tool/flutter_ci_script_beta.sh), and [master](tool/flutter_ci_script_master.sh), 

MAINTENANCE.md

@@ -38,7 +38,7 @@ If you have a general question about one of these samples or how to adapt its
 techniques for one of your own apps, try one of these resources:
 
 * [The FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
-* [The Flutter Gitter channel](https://gitter.im/flutter/flutter)
+* [The FlutterDev Discord](https://discord.gg/rflutterdev)
 * [StackOverflow](https://stackoverflow.com/questions/tagged/flutter)
 
 If you run into a bug in one of the samples, please file an issue in the