Remove "substrate-compat" default feature flag

[jsdw]

The flags we have enabled by default are ["jsonrpsee", "native", "substrate-compat"].

I'd very much like to remove "substrate-compat" from this default list; it leads to various issues when substrate crates are out of date (common atm since they aren't being published again yet), and pulls in a load of dependencies that aren't needed so much now we have subxt-signer.

So, can we remove it, and feature flag the code/examples that depend on it (and make sure we still do test those in CI)?

I recently found a handy trick for doc examples; you can add eg this around an example to make compiling it conditional on a feature flag:

//! ```rust
//! # // don't compile when feature flag not present:
//! # #[cfg(not(feature = "foo"))]
//! # fn main() {}
//! #
//! # #[cfg(feature = "foo")]
//! # fn main() {
//! 
//! // example requiring feature goes here
//!
//! # }

Not super pretty but I think there's only one/two doc examples using "substrate-compat" :)