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

Also:

golang/go@0d6e66c455 - net/http/internal/http2: don't use a per-Server error channel pool
https://go-review.googlesource.com/c/go/+/827524 - crypto/tls: don't pin record-sized buffers while blocked in Read
https://go-review.googlesource.com/c/go/+/828204 - net/http/internal/http2: turn Framer.getReadBuf func field into a method
https://go-review.googlesource.com/c/go/+/828504 - net/http: don't pin bufio buffers on idle HTTP/1 keep-alive connections
https://go-review.googlesource.com/c/go/+/828205 - net/http/internal/http2: don't pin frame-sized read buffers on idle conns
https://go-review.googlesource.com/c/go/+/828206 - net/http/internal/http2: don't pin frame-sized write buffers on idle conns

And

https://go.googlesource.com/go/+/5c51011e82ed754af072baaf4338ff35e76fe705
and
https://go.googlesource.com/go/+/0ac31b68d0787ed4a2b13a5dc35d3cba628e8bde
and
https://go-review.googlesource.com/c/go/+/833444
and
https://go.googlesource.com/go/+/a081f7d1fa8b9c29423583eccedfd1658ed66ef9
and
807969e