Move backfill: "false" out of properties, make it a boolean. (#167820)

Towards https://github.com/flutter/flutter/issues/167755.

Cocoon supports both formats at the moment, so this a NOP.

.ci.yaml

@@ -584,6 +584,7 @@ targets:
   - name: Linux docs_publish
     recipe: flutter/docs
     presubmit: false
+    backfill: false
     timeout: 60
     dimensions:
       os: "Linux"
@@ -602,7 +603,6 @@ targets:
         ]
       tags: >
         ["framework", "hostonly", "linux"]
-      backfill: "false"
       validation: docs
       validation_name: Docs
       firebase_project: main-docs-flutter-prod

engine/src/flutter/.ci.yaml

@@ -263,9 +263,9 @@ targets:
     recipe: engine_v2/engine_v2
     bringup: true
     timeout: 120
+    backfill: false
     properties:
       release_build: "false"
-      backfill: "false"
       config_name: linux_host_engine_ddm
       dependencies: >-
         [
@@ -305,10 +305,10 @@ targets:
     recipe: engine_v2/engine_v2
     bringup: true
     timeout: 120
+    backfill: false
     properties:
       config_name: linux_android_aot_engine_ddm
       release_build: "false"
-      backfill: "false"
     # Do not remove(https://github.com/flutter/flutter/issues/144644)
     # Scheduler will fail to get the platform
     drone_dimensions:
@@ -343,9 +343,9 @@ targets:
     recipe: engine_v2/engine_v2
     bringup: true
     timeout: 120
+    backfill: false
     properties:
       release_build: "false"
-      backfill: "false"
       config_name: linux_android_debug_engine_ddm
     # Do not remove(https://github.com/flutter/flutter/issues/144644)
     # Scheduler will fail to get the platform