net/http, x/net/http2: http server shutdown doesn't gracefully shut down HTTP2 connections [1.13 backport]

[gopherbot]

@networkimprov requested issue #39776 to be considered for backport to the next 1.13 minor release.

@gopherbot please backport to 1.13 & 1.14. Per @fraenkel this does not behave as documented and there is no workaround.

[networkimprov]

Some sites are not able to upgrade to 1.14 or 1.15 because async preemption breaks them (#40722 recently), and there's no way to disable it at build-time -- only the env variable at runtime.

[ianlancetaylor]

The Go project has a pretty clearly specified rule: we support two releases at a time. Of course anybody else is free to maintain patches for older releases. It's just not a burden we can take on ourselves.