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.library-conventions.gradle

@@ -0,0 +1,21 @@
+plugins {
+    id 'java-library'
+    id 'maven-publish'
+    id 'brouter.java-conventions'
+}
+
+publishing {
+    repositories {
+        maven {
+            name = 'GitHubPackages'
+            url = uri("https://maven.pkg.github.com/$System.env.REPO")
+            credentials {
+                username = project.findProperty('gpr.user') ?: System.getenv('USERNAME')
+                password = project.findProperty('gpr.key') ?: System.getenv('TOKEN')
+            }
+        }
+    }
+    publications {
+        gpr(MavenPublication)
+    }
+}