Bump httr2 requirement and remove old code

[hadley]

In a year or so, we can remove modern_httr2() and code in the FALSE branches.

[sckott]

@hadley Do you think we're at a point where we can do this change?

[hadley]

Yeah, I think so.

[sckott]

@hadley What were you thinking here? https://github.com/ropensci/vcr/blob/main/R/serialize-headers.R#L48 Remove what?

[hadley]

I think you could now use httr2::req_get_headers()