stable version v1.14.0

A new major version. Not so much has changed since the last of the 1.12
series, 1.12.15, but a bunch of small fixes and:

- updated requirements:
   - require C++20
   - require meson 1.3.2
   - require glib & friend: 2.80
   - require xapian 1.4.22
    - emacs 28.1 (for mu4e)
- warn for deprecated guile (use scm)
- a number of code-cleanups since we can use C++20 now

- indexing: substantial speed-up of the clean-up phase

- scm: support the --eval command-line option

- mu4e: improve message rendering; get rid of some the unnecessary
  body-part mime indication noise

- mu4e: allow including received patches when replying, see
  mu4e-compose-reply-include-mime-types

NEWS.org has more details.
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