chore(server/conn): backport `max_pending_accept_reset_streams()`

[cratelyn]

NB: this commit is based on the 0.14.x release series.

the server::conn::http2 submodule provides types that are conditionally compiled when the backports feature is active, to facilitate upgrading to the 1.x release.

i've been working on upgrading the linkerd2-proxy project to use hyper 1.0, but encountered a pertinent gap in Builder<E>'s interface when setting the backports and deprecated features.

this adds a max_pending_accept_reset_streams(..) method (added in #3201) to this builder, so that 0.14.x users relying on this functionality can prepare to upgrade to hyper 1.x safely.

this is effectively a backport of #3507, which restored this interface to the 1.0 release.

for more information, see:

• https://github.com/hyperium/hyper/pull/3201
• https://github.com/hyperium/hyper/pull/3507
• https://github.com/hyperium/hyper/issues/3461
• https://hyper.rs/guides/1/upgrading/

[seanmonstar]

Submitted #3797 to fix that MSRV job.

[cratelyn]

Submitted #3797 to fix that MSRV job.

thank you! i'll rebase now