net/http: enabling UnencryptedHTTP2 on Server disables first header read timeout [1.25 backport]

[gopherbot]

@neild requested issue #80205 to be considered for backport to the next 1.25 minor release.

@gopherbot please open backport issues for this security fix.

[gopherbot]

Change https://go.dev/cl/797820 mentions this issue: [release-branch.go1.25] net/http: apply header timeout to server's unencrypted HTTP/2 check

[gopherbot]

Closed by merging CL 797820 (commit 7841324) to release-branch.go1.25.