Release bit64 4.6.0

[MichaelChirico]

I've cleared the PR queue and handled some of the pending issues.

TODO:

• https://github.com/r-lib/bit64/milestone/1
• Get an old-R codespace (or 2) running & ensure we satisfy our stated R dependency
• Check URLs: urlchecker::url_check() or tools:::check_url_db(tools:::url_db_from_package_sources("."))
• Run the R-CMD-check GHA on all released R versions on Ubuntu, Mac, and Windows. (See [do not submit] check on all claimed R versions #109)
• devtools::check(remote = TRUE, manual = TRUE)
• devtools::check_win_devel(): https://win-builder.r-project.org/7wrJ3BeNVCQW/00install.out
• Run revdepcheck::revdep_check().
• On local CRAN release branch
  • Update version in DESCRIPTION
  • Finalize NEWS

[MichaelChirico]

Get an old-R codespace (or 2) running & ensure we satisfy our stated R dependency

#103 helped here, the codespace works and I did my best to run tests there. Looks good enough. Since we don't have {testthat}, it's actually harder than it looks to "prove" we satisfy the stated R dependency.