src/go.mod, net/http: update bundled and latest golang.org/x/net

Updates x/net/http2 to git rev 62affa334b73ec65ed44a326519ac12c421905e3

    x/net/http2: reject HTTP/2 Content-Length headers containing a sign
    https://go-review.googlesource.com/c/net/+/236098/ (fixes #39017)

also updates the vendored version of golang.org/x/net by running

go get golang.org/x/net@62affa334b73ec65ed44a326519ac12c421905e3
go mod tidy
go mod vendor
go generate -run bundle net/http

Change-Id: I7ecfdb7644574c44c3616e3b47664eefd4c926f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/253238


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>