Enable clippy::single_char_pattern (#8727)

This PR enables the
[`clippy::single_char_pattern`](https://rust-lang.github.io/rust-clippy/master/index.html#/single_char_pattern)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/collab/src/db/tests.rs

@@ -168,7 +168,7 @@ async fn new_test_user(db: &Arc<Database>, email: &str) -> UserId {
         email,
         false,
         NewUserParams {
-            github_login: email[0..email.find("@").unwrap()].to_string(),
+            github_login: email[0..email.find('@').unwrap()].to_string(),
             github_user_id: GITHUB_USER_ID.fetch_add(1, SeqCst),
         },
     )