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

tooling/xtask/src/main.rs

@@ -124,7 +124,6 @@ fn run_clippy(args: ClippyArgs) -> Result<()> {
         "clippy::redundant_locals",
         "clippy::reversed_empty_ranges",
         "clippy::search_is_some",
-        "clippy::single_char_pattern",
         "clippy::single_range_in_vec_init",
         "clippy::suspicious_to_owned",
         "clippy::to_string_in_format_args",