Add samplesBranch property

Closes gh-10019

gradle.properties

@@ -5,6 +5,7 @@ springFrameworkVersion=5.3.7
 openSamlVersion=3.4.6
 version=5.6.0-SNAPSHOT
 kotlinVersion=1.5.0
+samplesBranch=main
 org.gradle.jvmargs=-Xmx3g -XX:MaxPermSize=2048m -XX:+HeapDumpOnOutOfMemoryError
 org.gradle.parallel=true
 org.gradle.caching=true