Disable calls.share_on_join by default (#12401)

This PR changes the default value of the `calls.share_on_join` setting
from `true` to `false`.

Nathan mentioned that project sharing should be opt-in so that projects
aren't shared unless you intend for them to be.

Release Notes:

- Changed the default `calls.share_on_join` value to `false`.

docs/src/configuring-zed.md

@@ -1567,7 +1567,7 @@ Run the `theme selector: toggle` action in the command palette to see a current
   // Join calls with the microphone live by default
   "mute_on_join": false,
   // Share your project when you are the first to join a channel
-  "share_on_join": true
+  "share_on_join": false
 },
 ```