Automatically bump `mixxx` on 2.4 branch

[fwcd]

Since scheduled GitHub workflows unfortunately only run on the default branch, we'd have to modify main's workflow accordingly, e.g.

diff --git a/.github/workflows/update-mixxx.yml b/.github/workflows/update-mixxx.yml
index 0715f62..500cc06 100644
--- a/.github/workflows/update-mixxx.yml
+++ b/.github/workflows/update-mixxx.yml
@@ -9,10 +9,14 @@ on:
 jobs:
   automerge:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        branch: ['2.4', 'main']
 
     steps:
     - uses: actions/checkout@v3
       with:
+        ref: '${{ matrix.branch }}'
         fetch-depth: '0' # to compute the monotonic version correctly
         submodules: true
     - name: Configure Git user

The issue is that the build currently depends on a workflow_run trigger, for which I am not sure if it triggers in the way we would expect across branches: Given that the scheduled workflows run on main, 2.4's build workflow would likely not end up being triggered. We might be able to work around that by using e.g. a repository_dispatch event, but I am not sure if workflows on non-default branches will receive these events. This would be something to investigate.