acp: Change default for gemini back to managed version (#45218) (cherry-pick to stable) (#45224)

Cherry-pick of #45218 to stable

----
It seems we unintentionally changed the default behavior of if we use
the gemini on the path in #40663

Changing this back so by default we use a managed version of the CLI so
we can better control min versions and the like, but still allow people
to override if they need to.

Release Notes:

- N/A

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/project/src/agent_server_store.rs

@@ -459,7 +459,7 @@ impl AgentServerStore {
                     .gemini
                     .as_ref()
                     .and_then(|settings| settings.ignore_system_version)
-                    .unwrap_or(false),
+                    .unwrap_or(true),
             }),
         );
         self.external_agents.insert(