Update release source branch to 'main'

mkit.ini

     maintainer  = Alois Mahdal <netvor+imapdomo@vornet.cz>
     vcs_browser = https://gitlab.com/vornet/hacktop/imapdomo
     tagline     = imapfilter convenience wrapper
-    relsrc      = master
+    relsrc      = main
     reldst      = latest
 
 [dist]