build: bump version to 1.14.0

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.

meson.build

@@ -17,7 +17,7 @@
 ################################################################################
 # project setup
 project('mu', ['c', 'cpp'],
-        version: '1.14.0-pre3',
+        version: '1.14.0',
         meson_version: '>= 1.3.2',
         license: 'GPL-3.0-or-later',
         default_options : [
@@ -28,7 +28,7 @@ project('mu', ['c', 'cpp'],
 
 # hard-code the date here (for reproducibility); we derive the dates used in
 # e.g. documentation from this.
-mu_date='2026-02-24'
+mu_date='2026-03-28'
 
 # installation paths
 prefixdir = get_option('prefix')