Use conventions instead of cross-project configuration

gradle userguide suggests to avoid allprojects/subprojects and use conventions instead

https://docs.gradle.org/current/userguide/sharing_build_logic_between_subprojects.html#sec:convention_plugins_vs_cross_configuration

buildSrc/src/main/groovy/brouter.version-conventions.gradle

@@ -0,0 +1,7 @@
+// NOTE:
+// there are four places to change the version number
+// this file
+// app: build.gradle (versionCode only)
+// OsmTrack (version and versionDate)
+// docs revisions.md (version and versionDate)
+version '1.7.3'