Upgrade `bs_theme(version=4)` to 4.6.1

Hi,
as mentioned in issue #412 we try to package bslib for Debian (as a predependency for other packages like shiny). I've just mentioned that we need to provide the sources for compressed JS files. Alternatively we can link to just packaged code inside Debian which works nicely for bootstrap3 since its the very same version as in bslib. Unfortunately for bootstrap4 we have a minor discrepancy in the versioning. Debian has 4.6.1 while bslib ships 4.6.0. It turns out that the file _deprecated.scss was droped in version 4.6.1 which leads to the following test suite errors when I'm replacing bootstrap4 shipped with bslib by the Debian packaged version:

Error in `sass_file(f)`: Could not find file: '/usr/lib/R/site-library/bslib/lib/bs4/scss/./_deprecated.scss' in dir: /tmp/autopkgtest-lxc.re9ddi7e/downtmp/autopkgtest_tmp/tests/testthat

There is a full build log in our CI instance which shows 27 of these failures. I admit I have no idea why the code is exactly checking for that missing file. However, I wonder whether it would be the easiest solution if you would simply switch bslib to that bugfix release of bootstrap4. This would leave me only the task to deal with inst/lib/bs5/dist/js/bootstrap.bundle.min.js* (where I need to provide the uncompressed source to be acceptable for Debian distribution).

Kind regards, Andreas.

Start by inspecting the Bootstrap 4 assets under lib/bs4/scss, especially _deprecated.scss, and the checks in tests/testthat that reference it. Verify how bs_theme(version=4) selects and tests the bundled Bootstrap version, then run the test suite with Bootstrap 4.6.1. Done means the Bootstrap 4 tests pass with the updated asset set and the Debian packaging discrepancy is resolved.