Future-proof indexing on maps: remove IndexMut

This commit removes the `IndexMut` impls on `HashMap` and `BTreeMap`, in
order to future-proof the API against the eventual inclusion of an
`IndexSet` trait.

Ideally, we would eventually be able to support:

```rust
map[owned_key] = val;
map[borrowed_key].mutating_method(arguments);
&mut map[borrowed_key];
```

but to keep the design space as unconstrained as possible, we do not
currently want to support `IndexMut`, in case some other strategy will
eventually be needed.

Code currently using mutating index notation can use `get_mut` instead.

[breaking-change]

Closes #23448

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