Build is throwing deprecation warnings after Jekyll 4.3.4

[gonX]

After #325, the build is now throwing a bunch of deprecation warnings, such as:

DEPRECATION WARNING: Sass @import rules are deprecated and will be removed in Dart Sass 3.0.0.
DEPRECATION WARNING: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
DEPRECATION WARNING: darken() is deprecated.

et.al.

We need to fix these before we can keep upgrading packages.

[gonX]

Not correctly fixable until Bootstrap 5.5 unless we pin Sass version or silence deprecation warnings for libraries. See https://github.com/orgs/twbs/discussions/41370

[gonX]

Workaround added in #266 but blocks update of sass-converter until Bootstrap 5.5