Remove the golang.org/x/net/http2 dependency

[na--]

We use golang.org/x/net/http2 only for the ConfigureTransport() function. It is needed because when we use a non-nil TLSClientConfig in http.Transport, we break Go's automatic HTTP2 support and need ConfigureTransport() to fix it. Since we don't support old Go versions and since that's the only thing from golang.org/x/net/http2 we use, we may as well copy it into k6 and drop the dependency.

[na--]

Turns out this is more important than I realized! When using the http.ConfigureTransport() from golang.org/x/net/http2, it makes k6 use whatever version of that dependency we've configured in go.mod, and we'd forgotten to update that dependency for quite a while... 😞

So we don't have to do something like #1734 on every k6 release and Go version bump, we probably should figure out some way to depend only on the standard Go library, if that's at all possible...

[mstoykov]

We won't actually be able to drop it as it is also used by gRPC directly (for other reasons it seems). But maybe still, a good idea if we can make use of some black/unsafe magic to use the stdlib one instead.