CI: bump development builds explicitely to Ubuntu 24.04

Initially we agreed to split builds into "latest" for development branch
and fixed 22.04 for stable branches. It got broken when "latest" label migrated
from ubuntu-22 to ubuntu-24 ... because of build cache. Cache key is built using
runner label, it was not prepared to use the same "latest" cache from ubuntu 22
on ubuntu 24. To make things clear, let's stick explicitely to ubuntu 24.

Author: chipitsine

.github/matrix.py

@@ -96,7 +96,7 @@ def main(ref_name):
     if "haproxy-" in ref_name:
         os = "ubuntu-22.04" # stable branch
     else:
-        os = "ubuntu-latest" # development branch
+        os = "ubuntu-24.04" # development branch
 
     TARGET = "linux-glibc"
     for CC in ["gcc", "clang"]: