Add Spring libs-snapshot Repository declaration to Gradle dependency management in order to resolve the 'provided' Spring Boot dependency.

This will enable Spring Boot build snapshots and milestone artifacts (e.g. M1, RC1, etc) to be resolved from the Spring repos for STDG GA releases.

While this is generally unacceptable per release policy, since only GA artifacts should ship with GA artifacts, the Spring Boot dependency in STDG is declared 'provided' and therefore is not materialized in the final STDG Maven POM published to Maven Central.

Author: jxblum

gradle/dependency-management.gradle

@@ -1,3 +1,9 @@
+repositories {
+	maven {
+		url "https://repo.spring.io/libs-snapshot"
+	}
+}
+
 dependencyManagement {
 	imports {
 		mavenBom "org.springframework.boot:spring-boot-dependencies:$springBootVersion"