Update modus-themes to their version 2.3.0

Update modus-themes to their version 2.3.0

* doc/misc/modus-themes.org (Enable and load): Clarify wording.
(Sample configuration with and without use-package): Improve sample
code on how to set up the themes.
(Customization Options): Update sample configuration.
(Option for box buttons, Option for mode line presentation)
(Option for completion framework aesthetics)
(Option for Org agenda constructs)
(Option for the headings' overall style): Document how to optionally
pass number values as a cons cell.
(Option for mouseover effects): Document new boolean user option.
(More accurate colors in terminal emulators): Write about the color
range in terminal emulators and provide sample palette for XTerm.
(Override colors): Use American English.
(Near-monochrome syntax highlighting): Provide sample code on how to
achieve a monochrome style.
(Full support for packages or face groups)
(Indirectly covered packages): Update lists of supported packages.
(Note on display-fill-column-indicator-mode): Reword node.
(Note on prism.el): Use American English.
(Note on SHR colors): Clarify statement.
(Note on the Notmuch logo): Remark that the Notmuch logo can be
disabled.
(Port the Modus themes to other platforms?): Use American English.
(Sources of the themes): Fix capitalization of proper nouns.
(Acknowledgements): Update list of contributors to the project.

* etc/themes/modus-operandi-theme.el:
* etc/themes/modus-vivendi-theme.el: Ensure that the theme is reified
as expected both at compiletime and runtime.

* etc/themes/modus-themes.el (require): Require 'cl-lib' and 'subr-x'
at compiletime.
(seq): Require the 'seq' library.
(modus-themes-completion-standard-first-match)
(modus-themes-completion-standard-selected)
(modus-themes-completion-extra-selected): Use correct symbol for
deprecated faces.
(modus-themes-slanted-constructs): Provide it as an alias of
'modus-themes-italic-constructs'.
(modus-themes-variable-pitch-headings): Remove obsolete user option.
(modus-themes-no-mixed-fonts): Remove obsolete user option alias.
(modus-themes-intense-mouseovers): Add new user option.
(modus-themes--headings-choice): Accept value as a cons cell.
(modus-themes-headings, modus-themes-org-agenda): Update user option
to accept number value as a cons cell.
(modus-themes-scale-headings, modus-themes-scale-1, modus-themes-scale-2)
(modus-themes-scale-3, modus-themes-scale-4, modus-themes-scale-title)
(modus-themes-scale-small): Remove obsolete user options.
(modus-themes-mode-line): Update user option to accept number values
as cons cells.
(modus-themes-mode-line-padding): Remove obsolete user option.
(modus-themes-completions): Add support for the 'text-also' property
and update it accordingly.
(modus-themes-success-deuteranopia): Remove obsolete user option.
(modus-themes-box-buttons): Update user option to accept number values
as cons cells.
(modus-themes--warn, modus-themes--list-or-warn)
(modus-themes--alist-or-seq): Add functions to check for correct value
in some user options.
(modus-themes--current-theme): Return the first Modus theme from
'current-enable-themes' (bug#54598).
(modus-themes--lang-check, modus-themes--prompt, modus-themes--paren)
(modus-themes--syntax-foreground, modus-themes--syntax-extra)
(modus-themes--syntax-string, modus-themes--syntax-comment)
(modus-themes--heading, modus-themes--agenda-structure)
(modus-themes--agenda-date, modus-themes--mode-line-attrs)
(modus-themes--completion, modus-themes--link, modus-themes--link-color)
(modus-themes--region, modus-themes--hl-line, modus-themes--button):
Make private functions check for the desired value.  Refine them where
necessary.
(modus-themes-faces, modus-themes-custom-variables): Update supported
faces and relevant variables.