runtime: shorten default thread name to fit in Linux limit (#7880)

Linux thread names are truncated at 15 characters.
Currently, Tokio threads show up as "tokio-runtime-w", whereas
shortening "runtime" to "rt" would make the thread name perfectly fit
in the 15 character limit.

tokio/src/runtime/builder.rs

@@ -288,7 +288,7 @@ impl Builder {
             max_blocking_threads: 512,
 
             // Default thread name
-            thread_name: std::sync::Arc::new(|| "tokio-runtime-worker".into()),
+            thread_name: std::sync::Arc::new(|| "tokio-rt-worker".into()),
 
             // Do not set a stack size by default
             thread_stack_size: None,