[maven-release-plugin] prepare release ghprb-1.28.6
[maven-release-plugin] prepare release ghprb-1.28.6

arcivanov/ghprb-plugin,ithaka/ghprb-plugin,sradi/ghprb,AtkinsChang/ghprb,arcadia/ghprb-plugin,fbelzunc/ghprb-plugin,jenkinsci/ghprb-plugin,dulvac/ghprb-plugin,jenkinsci/ghprb-plugin,DavidTanner/ghprb-plugin,dulvac/ghprb-plugin,janinko/ghprb,mmitche/ghprb,arcadia/ghprb-plugin,DavidTanner/ghprb-plugin,sradi/ghprb,mmitche/ghprb,ithaka/ghprb-plugin,janinko/ghprb,AtkinsChang/ghprb,arcivanov/ghprb-plugin,fbelzunc/ghprb-plugin