net/http: close connections when receiving too many headers (CVE-2023-45288) [1.21 backport]

[gopherbot]

@neild requested issue #65051 to be considered for backport to the next 1.21 minor release.

@gopherbot please open backport issues for this security fix.

[neild]

Deferred pending coordinated disclosure, will reopen when we know what release this goes into.