Replace "coin" with "network"

The Processor's coins folder referred to the networks it could process, as did
its Coin trait. This, and other similar cases throughout the codebase, have now
been corrected.

Also corrects dated documentation for a key pair is confirmed under the
validator-sets pallet.

substrate/client/src/lib.rs

@@ -1,5 +1,5 @@
-#[cfg(feature = "coins")]
-pub mod coins;
+#[cfg(feature = "networks")]
+pub mod networks;
 
 #[cfg(feature = "serai")]
 mod serai;

substrate/client/src/tests/mod.rs

@@ -1,2 +1,2 @@
-#[cfg(feature = "coins")]
-mod coins;
+#[cfg(feature = "networks")]
+mod networks;