Remove outdated dependencies; use testthat-3e

[jcheng5]

See #833

The code change in normalize-sp.R is assuming that r-spatial/sf#2069 gets merged. If not, either we'll need similar code, or else just warn/error that we don't handle this edge case. In either case, I need to write a test that exercises that code path.

PR task list:

• Update NEWS
• Add tests (where appropriate)
  • R code tests: tests/testthat/
  • Visual tests: R/zzz_viztest.R
• Update documentation with devtools::document()

[jcheng5]

@gadenbuie I added the unit test we discussed today; this is now ready to go. However, the rare case we discussed (sp objects that have holes but are missing the comment that indicates hole ownership) only works with sf 1.0-10, which is not yet on CRAN. If this case is encountered while the CRAN version of sf is installed, leaflet will report an error telling you to upgrade sf to 1.0-10 or above. The tests will pass either way, as I don't attempt the unit test if the wrong version of sf is installed.

I don't think we have to wait for sf 1.0-10 to hit CRAN to merge this PR, but I do think we should wait for that before we submit leaflet to CRAN.