Drop the default buffer size to 8K

Drop the default buffer size to 8K

The 64k capacity was picked by me a couple of years ago in the initial
implementation of buffered IO adaptors:
https://github.com/rust-lang/rust/pull/9091/files#diff-b131eeef531ad098b32f49695a031008R62.
64K was picked for symmetry with libuv, which we no longer use.

64K is *way* larger than the default size of any other language that I
can find. C, C++, and Java default to 8K, and Go defaults to 4K. There
have been a variety of issues filed relating to this such as #31885.

Closes #31885