Fix slow query for fetching descendants of channels (#7008)

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

crates/collab/migrations/20240129193601_fix_parent_path_index.sql

@@ -0,0 +1,4 @@
+-- Add migration script here
+
+DROP INDEX index_channels_on_parent_path;
+CREATE INDEX index_channels_on_parent_path ON channels (parent_path text_pattern_ops);