Add Gloas boilerplate (#7728)

Adds the required boilerplate code for the Gloas (Glamsterdam) hard fork. This allows PRs testing Gloas-candidate features to test fork transition.

This also includes de-duplication of post-Bellatrix readiness notifiers from #6797 (credit to @dapplion)

beacon_node/network/src/subnet_service/tests/mod.rs

@@ -94,10 +94,9 @@ pub fn recent_genesis_time() -> u64 {
 
 fn get_tracing_subscriber(log_level: Option<&str>) {
     if let Some(level) = log_level {
-        tracing_subscriber::fmt()
+        let _ = tracing_subscriber::fmt()
             .with_env_filter(EnvFilter::try_new(level).unwrap())
-            .try_init()
-            .unwrap();
+            .try_init();
     }
 }