Pull Modus themes version 1.2.3 from upstream

Pull Modus themes version 1.2.3 from upstream

This syncs with the following upstream revision:

Update to version 1.2.3
0a36239 2021-03-05 19:43:30 +0200
https://gitlab.com/protesilaos/modus-themes/-/commit/0a36239baf908585cdf32c6188eb86713d9bf6c6

For discussion, see bug#45068 and the following upstream issue:
https://gitlab.com/protesilaos/modus-themes/-/issues/162

* doc/misc/modus-themes.org:
* etc/themes/modus-operandi-theme.el:
* etc/themes/modus-themes.el:
* etc/themes/modus-vivendi-theme.el: Update to version 1.2.3.