collab: Seed GitHub users from static data (zed-industries#18301)

This PR updates the collab seed script to seed the GitHub users from a
set of static data.

This removes the need to hit the GitHub API to retrieve these users.

Release Notes:

- N/A

Author: maxdeviant

crates/collab/README.md

@@ -23,8 +23,7 @@ To use a different set of admin users, create `crates/collab/seed.json`.
 ```json
 {
   "admins": ["yourgithubhere"],
-  "channels": ["zed"],
-  "number_of_users": 20
+  "channels": ["zed"]
 }
 ```
 

crates/collab/seed.default.json

@@ -8,6 +8,5 @@
     "JosephTLyons",
     "rgbkrk"
   ],
-  "channels": ["zed"],
-  "number_of_users": 100
+  "channels": ["zed"]
 }