Prep work for branch renaming (#389)

* Add `amd-staging` and `amd-mainline` to workflow.
* Update branch names in documentation
* Change `dev` to `amd-staging` and `main` to `amd-mainline` in documentation.
* Remove references from 2.x from workflows.
* Convert the link to LICENSE to a relative path in CONTRIBUTING.

---------

Signed-off-by: David Galiffi <David.Galiffi@amd.com>

.github/workflows/packaging.yml

@@ -8,7 +8,7 @@ on:
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
-  
+
 jobs:
   distbuild:
     runs-on: ubuntu-latest