Do not automatically backport each PR to stable

The way we have been doing releases does not coincide well with how we
have been utilizing our stable branch. Given that we release from
master, and that many people in the community use master on a daily
basis, it makes more sense for stable to always match the latest
released version, and for master to include unreleased changes which are
considered "stable".

For experiments, we should instead use a feature branch per experiment
until we are confident that it is ready to be merged into master.

.mergify.yml

@@ -1,5 +1,5 @@
 pull_request_rules:
-  - name: automatic merge for main when CI passes and approved
+  - name: automatic squash-merge for master when CI passes and approved
     conditions:
       - "#approved-reviews-by>=1"
       - and:
@@ -15,8 +15,9 @@ pull_request_rules:
         method: squash
         rebase_fallback: none
         update_method: rebase
-  - name: automatic merge for stable backports when CI passes
+  - name: automatic merge for stable when CI passes and approved
     conditions:
+      - "#approved-reviews-by>=1"
       - and:
           # Unfortunately, we cannot just say "all CI must pass".
           # This is not a bot limitation, but a limitation in Github itself.
@@ -24,33 +25,12 @@ pull_request_rules:
           - check-success=docker
           - check-success=SonarCloud Code Analysis
       - base=stable
-      - or:
-          - "#approved-reviews-by>=1"
-          - author=mergify[bot]
     actions:
       queue:
         name: default
         method: rebase
-        rebase_fallback: none
+        rebase_fallback: merge
         update_method: rebase
-  - name: copy patches to stable branch
-    conditions:
-      - base=master
-      - label!=unstable
-    actions:
-      backport:
-        branches:
-          - stable
-  - name: assign code owners to stable merge conflicts
-    conditions:
-      - conflict
-      - base=stable
-    actions:
-      assign:
-        add_users:
-          - master-of-zen
-          - redzic
-          - shssoichiro
 
 queue_rules:
   - name: default