Bump default serialization version in use_data() to 3

[laurabrianna]

Fixes #1966 - The grand sum of this update is one character (plus a news bullet) Note that this change did not break anything in any test (should a test be added?)

[jennybc]

Looking good!

Can you take a look at the docs for the version argument? They state that the default is 2, which is not correct anymore. So this needs a refresh.

#' @param version The serialization format version to use. The default, 2, was
#'   the default format from R 1.4.0 to 3.5.3. Version 3 became the default from
#'   R 3.6.0 and can only be read by R versions 3.5.0 and higher.

[jennybc]

I think we do have a test that claims to probe default behaviour but it's clearly not working! Because it actually provides the version argument.

Would you also like to fix and update this test?

test_that("use_data() writes version 2 by default", {
  create_local_package()

  x <- letters
  use_data(x, internal = TRUE, version = 2, compress = FALSE)
  expect_identical(
    rawToChar(readBin(proj_path("R", "sysdata.rda"), n = 4, what = "raw")),
    "RDX2"
  )
})

[laurabrianna]

Hi Jenny thanks for all your help (and patience!) I incorporated both your notes above!

[jennybc]

Thanks!