Add deprecation notice to cranelift_use_egraphs option. (#5610)

After #5587, this is on by default. We are retaining the traditional
(no-egraphs) path for now, selected by setting this option to `false`,
but we eventually plan to delete it assuming that we don't find serious
regressions or issues. This PR adds a deprecation notice to the option.

crates/fuzzing/src/generators/config.rs

@@ -141,6 +141,7 @@ impl Config {
     }
 
     /// Converts this to a `wasmtime::Config` object
+    #[allow(deprecated)] // Allow use of `cranelift_use_egraphs` below.
     pub fn to_wasmtime(&self) -> wasmtime::Config {
         crate::init_fuzzing();
         log::debug!("creating wasmtime config with {:#?}", self.wasmtime);