configure.ac: Switch project version to 20.x.y style

Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
5 jobs for autotools-version-style in 4 minutes and 26 seconds
Status Job ID Name Coverage
  Docker Image
passed debian-buster #11206363

00:00:27

 
  Build And Test
passed autotools #11206364

00:03:58

passed meson #11206365

00:01:52

passed meson-noglamor #11206366

00:01:23

passed mingw-cross-build #11206367

00:00:36