Rollup merge of #126830 - RalfJung:unsized-fn-params, r=compiler-errors

make unsized_fn_params an internal feature

As suggested [here](#123894 (comment)).
r? `@compiler-errors`

Fixes #123887 (kind of -- ICEs on internal features are considered acceptable so this issue is not-a-bug once this PR lands)

compiler/rustc_feature/src/unstable.rs

@@ -629,7 +629,7 @@ declare_features! (
     /// Allows unsafe on extern declarations and safety qualifiers over internal items.
     (unstable, unsafe_extern_blocks, "1.80.0", Some(123743)),
     /// Allows unsized fn parameters.
-    (unstable, unsized_fn_params, "1.49.0", Some(48055)),
+    (internal, unsized_fn_params, "1.49.0", Some(48055)),
     /// Allows unsized rvalues at arguments and parameters.
     (incomplete, unsized_locals, "1.30.0", Some(48055)),
     /// Allows unsized tuple coercion.