fix(mergify): put PRs with no priority label in the low priority queue (#3454)

This also lets us handle dependabot PRs using the low queue rule.

.github/mergify.yml

@@ -64,25 +64,13 @@ pull_request_rules:
       - "#review-threads-unresolved=0"
       - -draft
       - base=main
-      - or:
-          - "label~=^P-Low"
-          - "label~=^P-Optional"
       - label!=do-not-merge
+      # This queue handles Low, Optional, and PRs with no 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
     actions:
       queue:
         name: low
         method: squash
-
-  - name: automatic merge for Dependabot pull requests
-    conditions:
-      - "#approved-reviews-by>=1"
-      - "#review-threads-unresolved=0"
-      - author~=^dependabot(|-preview)\[bot\]$
-      - check-success=Test (+stable) on ubuntu-latest
-      - check-success=Test (+stable) on macOS-latest
-      - check-success=Test (+stable) on windows-latest
-      - check-success=pull-request (zealous-zebra)
-      - check-success=Coverage (+nightly)
-    actions:
-      merge:
-        method: squash