[CP-beta]Move backfill: "false" out of properties, make it a boolean. (#167831)

This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/blob/main/docs/releases/Flutter-Cherrypick-Process.md#automatically-creates-a-cherry-pick-request)
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### Issue Link:
What is the link to the issue this cherry-pick is addressing?

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

### Changelog Description:
Explain this cherry pick in one line that is accessible to most Flutter developers. See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples

N/A (CI Infrastructure Only)

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)

Allows removing technical debt from Cocoon.

### Workaround:
Is there a workaround for this issue?

N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

It is already live on the master channel.

.ci.yaml

@@ -585,6 +585,7 @@ targets:
   - name: Linux docs_publish
     recipe: flutter/docs
     presubmit: false
+    backfill: false
     timeout: 60
     dimensions:
       os: "Linux"
@@ -603,7 +604,6 @@ 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