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.
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