Remove sourceMappingURL from CSS files to fix #8587.

[donkirkby]

Fixes a problem with Django 4.1, refs #8587.

This is the smallest change, just removing the two sourceMappingURL lines from the minified CSS files. One downside, however, is that someone upgrading to a new version of bootstrap might accidentally add them back in. Maybe a precommit hook could guard against that.

Another option would be to include the source maps, but I thought that was a bigger change.