net/http/pprof: avoid panic with user-defined "GET /" route

[xpmatteo]

With the new routing style in go 1.22, declaring

http.Handle("GET /", h)

generates a conflict with route "/debug/pprof/" and the others declared in
the net/http/pprof package. You get an error such as:

panic: pattern "GET /" (registered at .../pprof.go:94): GET / matches
fewer methods than /debug/pprof/, but has a more general path pattern

This patch prevents that error. Adding GET is correct because no other
method makes sense with the /debug/pprof routes. However, a tool using any
method other than GET will break.

We preserve the traditional behaviour when GODEBUG=httpmuxgo121=1 is
specified.

Updates #65723

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