Incorrect license in forked code, should be MIT-only

[teor2345]

Hi, I'm one of the Zcash Foundation developers who helped write the original version of the tower-batch code.

We just found out we never had permission to release our tower-batch code under the Apache License 2.0. We originally forked the code from tower, which is under the MIT license only:
https://github.com/tower-rs/tower/tree/master/tower/src/buffer

So that means any code from tower-buffer that's in your tower-batch can only be released under the MIT license, with these copyright notices:

Copyright (c) 2019 Tower Contributors
Copyright (c) 2019-2023 Zcash Foundation
Copyright (c) 2022-2023 chmodas

Sorry about that, we just realised it had happened a few days ago.

Here is our PR to change our licenses:
ZcashFoundation/zebra#6927