Auto merge of #113224 - zachs18:vec_extend_remove_allocator_lifetime, r=cuviper

Remove lifetime bound for A for `impl Extend<&'a T> for Vec<T, A>`.

The lifetime of the references being copied from is unrelated to the allocator.

Compare with [`impl<'a, T: 'a + Copy, A: Allocator> Extend<&'a T> for VecDeque<T, A>`](https://doc.rust-lang.org/alloc/collections/vec_deque/struct.VecDeque.html#impl-Extend%3C%26'a+T%3E-for-VecDeque%3CT,+A%3E) which does not have the `A: 'a` bound already.

Since `Allocator` is unstable, the only possible `A` on stable is `Global`, and `Global: 'static`, so this change is not (should not be) observable on stable (or without `#![feature(allocator_api)]`). [This is observable on nightly](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=8c4aa166c6116a90593d2934d30cfeb3).

Author: bors