Fix copyright-fix-years, use a dash to separate years in a range

Fix copyright-fix-years, use a dash to separate years in a range

Commit 7a46fa9c751f7c3147cd9b27fe84c1ce25419d3b, allowing en-dash as
years separator, changed the way the years range is composed taking the
separator from the `sep' variable instead of always using an hardwired
hyphen. Since the code explicitly avoid to touch (that is, extend)
existing ranges, that `sep' variable is almost always a space, or
whatever character precedes the last parsed year, thus generating a
range like "2020 2022" instead of "2020-2022".

* lisp/emacs-lisp/copyright.el (copyright-fix-years): Always use an dash
to separate years in a range.

* test/lisp/emacs-lisp/copyright-tests.el (with-copyright-fix-years-test):
(copyright-fix-years-tests--data):
(text-copyright-fix-years): New test cases (bug#53597).