build: bump to 1.14.0-pre1, bump some required versions

Bump the required versions of some libraries / emacs slightly so we can get rid
of some special-casing and work-arounds. These are mostly based on Ubuntu 24.04
LTS, which is rather conservative; so should hopefully not inconvenience any
users.

- compiler: we now require C++20 instead of C++17
- glib & friends: 2.60 => 2.80
- xapian: 1.4 => 1.4.22
- emacs 26.3 => 28.1

Ubuntu 24.04 actually has Emacs 29.3, but that'd perhaps be too big a jump.

lib/mu-xapian-db.hh

@@ -33,8 +33,10 @@
 #include <utils/mu-result.hh>
 #include <utils/mu-utils.hh>
 
-/* starting with 1.4.6, Xapian supports C++ move semantics,
- * but only with XAPIAN_MOVE_SEMANTICS defined
+/*
+ * starting with 1.4.6, Xapian supports C++ move semantics,
+ * but only with XAPIAN_MOVE_SEMANTICS defined. We require
+ * at lest 1.4.22 so we can define it.
  */
 #ifndef XAPIAN_MOVE_SEMANTICS
 #define XAPIAN_MOVE_SEMANTICS