Rollup merge of rust-lang#126830 - RalfJung:unsized-fn-params, r=comp…

…iler-errors

make unsized_fn_params an internal feature

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

Fixes rust-lang#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.