Move dartdocs to devicelab vm. (#45952)

We added support linux VMs to devicelab and we will be moving some of
the machine independent benchmarks there.

dev/devicelab/manifest.yaml

@@ -624,7 +624,7 @@ tasks:
     description: >
       Tracks how many members are still lacking documentation.
     stage: devicelab
-    required_agent_capabilities: ["linux/android"]
+    required_agent_capabilities: ["linux-vm"]
 
   technical_debt__cost:
     description: >