Finish migration to role instead of is_admin (#9414)

Release Notes:

- Fixed a bug signing in for five users
([#4323](https://github.com/zed-industries/zed/issues/4323)).

crates/collab/migrations/20240315182903_non_null_channel_role.sql

@@ -0,0 +1,4 @@
+-- Add migration script here
+
+ALTER TABLE channel_members ALTER role SET NOT NULL;
+ALTER TABLE channel_members DROP COLUMN admin;

crates/collab/migrations/20240315183903_channel_parent_path_not_null.sql

@@ -0,0 +1,2 @@
+-- Add migration script here
+ALTER TABLE channels ALTER parent_path SET NOT NULL;