Update modus-themes to version 1.6.0

Update modus-themes to version 1.6.0

* doc/misc/modus-themes.org (Customization Options): Document new user
options in code sample.
(Option for inhibiting theme reload): Remove trailing space.
(Option for mode line presentation): Include new possible value.
(Option for Org agenda constructs): Include new symbols for the
'modus-themes-org-agenda' alist.
(Control the scale of headings): Document 'modus-themes-scale-small'.
(Override color saturation (DIY)): Fix markup for proper texi output.
(Custom Org user faces (DIY)): Update code samples.
(Full support for packages or face groups): Note names of newly
supported packages or face groups.
(Indirectly covered packages): Document indirectly supported packages.
(Note on highlight-parentheses.el): Provide detailed instructions on
how to use 'highlight-parentheses' with the Modus themes.
(Note on prism.el): Refine code samples.
(What is the best setup for legibility?): Minor rewording.
(Sources of the themes): Mention only Emacs28 without explaining that
it is the development target---in preparation of the emacs-28 branch
cut.
(Acknowledgements): Name new contributors to code/ideas.  Stephen
Gildea's patch was a couple lines long.  The others have assigned
copyright to the FSF.
(Meta): Include another link to the development notes of the themes
about 'modus-themes-org-agenda'.

* etc/themes/modus-operandi-theme.el,
  etc/themes/modus-vivendi-theme.el: Bump file version.

* etc/themes/modus-themes.el (modus-themes-operandi-colors)
(modus-themes-vivendi-colors): Recalibrate some colour values and add
a few new ones.
(modus-themes-slanted-constructs): Remove obsolete user option.
Superseded by the alias 'modus-themes-italic-constructs'.
(modus-themes-org-agenda, modus-themes-mode-line): Update user option.
(modus-themes-scale-headings, modus-themes-scale-4): Update doc
string.
(modus-themes-scale-5): Remove obsolete user option.  Superseded by
the alias 'modus-themes-scale-title'.
(modus-themes-scale-small, modus-themes-tabs-accented): Add new user
option.
(modus-themes--agenda-date, modus-themes--mode-line-attrs)
(modus-themes--tab): Update internal functions.
(modus-themes-faces): Update variousface attributes.

Detailed change log here:
<https://protesilaos.com/codelog/2021-09-29-modus-themes-1-6-0/>.