Add spring-milestone plugin repo for stagingmilestone profile

Author: onobc

.github/settings.xml

@@ -37,7 +37,15 @@
 						<enabled>false</enabled>
 					</snapshots>
 				</pluginRepository>
-			</pluginRepositories>
+				<pluginRepository>
+					<id>spring-milestones</id>
+					<name>Spring Milestones</name>
+					<url>https://repo.spring.io/libs-milestone</url>
+					<snapshots>
+						<enabled>false</enabled>
+					</snapshots>
+				</pluginRepository>				
+			</pluginRepositories>	
 		</profile>
 		<profile>
 			<id>stagingrelease</id>