Allow test to use snapshot/milestone repo

Update `spring-boot-maven-plugin` `settings.xml` so that milestone and
snapshot dependencies can be resolved.

See gh-23936

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/projects/settings.xml

@@ -18,6 +18,19 @@
 						<enabled>true</enabled>
 					</snapshots>
 				</repository>
+				<repository>
+					<id>spring-milestones</id>
+					<name>Spring Milestones</name>
+					<url>https://repo.spring.io/milestone</url>
+				</repository>
+				<repository>
+					<id>spring-snapshots</id>
+					<name>Spring Snapshots</name>
+					<url>https://repo.spring.io/snapshot</url>
+					<snapshots>
+						<enabled>true</enabled>
+					</snapshots>
+				</repository>
 			</repositories>
 			<pluginRepositories>
 				<pluginRepository>