patch: net/http/internal/http2: memory optimizations

[bradfitz]

Cherry-picking two Go 1.28 optimizations into our Go 1.27 tree:

https://go-review.googlesource.com/c/go/+/810780 - net/http/internal/http2: let Transport's request-write goroutine exit early
https://go-review.googlesource.com/c/go/+/811680 - net/http/internal/http2: release handler write buffers when idle