Migrate to {httr2}

[Moohan]

I've thought about this for a while, but never raised it as it feels too 'big'. However, on closer inspection, we don't actually use {httr} in too many places; it's pretty much just in the internal phs_GET function.

Reasons

• httr has been superceeded by httr2 for a while - https://httr.r-lib.org/
• This means it's not getting any new features, improvements or bug fixes, but it will stay on CRAN
• httr2 has a nice article on 'wrapping APIs', so it should be simple enough to follow their best practice - https://httr2.r-lib.org/articles/wrapping-apis.html