Merge associated_const_equality feature gate into MGCA

This removes `associated_const_equality` as a separate feature gate and makes it part of `min_generic_const_args` (mgca).

Key changes:
  - Remove `associated_const_equality` from unstable features, add to removed
  - Update all test files to use `min_generic_const_args` instead
  - Preserve the original "associated const equality is incomplete" error message by specially handling `sym::associated_const_equality` spans in `feature_gate.rs`
  - Rename FIXME(associated_const_equality) to FIXME(mgca)

Author: AprilNEA

crates/hir-ty/src/lower.rs

@@ -862,7 +862,7 @@ impl<'db, 'a> TyLoweringContext<'db, 'a> {
                                 TermKind::Ty(ty) => {
                                     ty.walk().any(|arg| arg == dummy_self_ty.into())
                                 }
-                                // FIXME(associated_const_equality): We should walk the const instead of not doing anything
+                                // FIXME(mgca): We should walk the const instead of not doing anything
                                 TermKind::Const(_) => false,
                             };