change(ci): Combine high and low Mergify queues into a batched queue (#5201)

* Combine high and medium queues into a batched queue

* Explain how to check config syntax

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.github/mergify.yml

@@ -1,5 +1,10 @@
+# Automatic merging of approved PRs for Zebra
+#
+# This file can be edited and validated using:
+# https://dashboard.mergify.com/github/ZcashFoundation/repo/zebra/config-editor
+
 queue_rules:
-  - name: hotfix
+  - name: urgent
     # Allow to update/rebase the original pull request if possible to check its mergeability,
     # and it does not create a draft PR if not needed
     allow_inplace_checks: True
@@ -14,28 +19,18 @@ queue_rules:
       # https://docs.mergify.com/conditions/#about-branch-protection
       - base=main
 
-  - name: high
+  - name: batched
     allow_inplace_checks: True
     allow_checks_interruption: True
     speculative_checks: 1
     batch_size: 20
-    # Wait for a few minutes to embark high priority tickets together in a merge train
-    batch_max_wait_time: "10 minutes"
-    conditions:
-      - base=main
-
-  - name: low
-    allow_inplace_checks: True
-    allow_checks_interruption: True
-    speculative_checks: 1
-    batch_size: 20
-    # Wait a bit longer to embark low priority tickets together in a merge train
+    # Wait for about 10% of the time it takes Rust PRs to run CI (3h)
     batch_max_wait_time: "20 minutes"
     conditions:
       - base=main
 
 pull_request_rules:
-  - name: move to hotfix queue when CI passes with 1 review and not WIP targeting main
+  - name: move to urgent queue when CI passes with 1 review and not WIP targeting main
     conditions:
       # This queue handles a PR if:
       # - it targets main
@@ -48,41 +43,24 @@ pull_request_rules:
       - 'label~=^P-Critical'
     actions:
       queue:
-        name: hotfix
+        name: urgent
         method: squash
 
-  - name: move to high queue when CI passes with 1 review and not WIP targeting main
+  - name: move to batched queue when CI passes with 1 review and not WIP targeting main
     conditions:
       # This queue handles a PR if:
       # - it targets main
       # - is not in draft
       # - does not include the do-not-merge label
-      # - is labeled with High or Medium priority
-      - base=main
-      - -draft
-      - label!=do-not-merge
-      - or:
-          - 'label~=^P-High'
-          - 'label~=^P-Medium'
-    actions:
-      queue:
-        name: high
-        method: squash
-
-  - name: move to low queue when CI passes with 1 review and not WIP targeting main
-    conditions:
-      # We don't need to check priority labels here, because the rules are evaluated in order:
-      # https://docs.mergify.com/configuration/#pull-request-rules
-      #
-      # This queue handles a PR if:
-      # - it targets main
-      # - is not in draft
-      # - is labeled with Low or Optional priority, or does not have a priority label,
+      # - is labeled with any other priority except Critical, or does not have a priority label,
       #   including automated dependabot PRs.
+      #
+      # We don't need to check priority labels here, because the rules are evaluated in order:
+      # https://docs.mergify.com/configuration/#pull-request-rules      
       - base=main
       - -draft
       - label!=do-not-merge
     actions:
       queue:
-        name: low
+        name: batched
         method: squash