Snowbridge Ethereum Deneb fork preparation (#3029)

- Prepares for the Deneb hardfork on Sepolia testnet on 31 January
(needs to be deployed to Rococo before then)
- Removes `beacon-minimal-spec` flag for simpler config
- Adds test comments

---------

Co-authored-by: Ron <yrong1997@gmail.com>
Co-authored-by: claravanstaden <Cats 4 life!>
Co-authored-by: Alistair Singh <alistair.singh7@gmail.com>

cumulus/parachains/runtimes/bridge-hubs/bridge-hub-rococo/Cargo.toml

@@ -294,9 +294,7 @@ try-runtime = [
 ]
 
 experimental = ["pallet-aura/experimental"]
-fast-runtime = [
-	"snowbridge-pallet-ethereum-client/beacon-spec-minimal",
-]
+fast-runtime = []
 
 # A feature that should be enabled when the runtime should be built for on-chain
 # deployment. This will disable stuff that shouldn't be part of the on-chain wasm