Auto merge of #90207 - BoxyUwU:stabilise_cg_defaults, r=lcnr

Stabilise `feature(const_generics_defaults)`

`feature(const_generics_defaults)` is complete implementation wise and has a pretty extensive test suite so I think is ready for stabilisation.

needs stabilisation report and maybe an RFC 😅

r? `@lcnr`
cc `@rust-lang/project-const-generics`

src/test/rustdoc/const-generics/const-generic-defaults.rs

@@ -1,5 +1,4 @@
 #![crate_name = "foo"]
-#![feature(const_generics_defaults)]
 
 // @has foo/struct.Foo.html '//pre[@class="rust struct"]' \
 //      'pub struct Foo<const M: usize = 10_usize, const N: usize = M, T = i32>(_);'