Create extension to configure Spring maven repositories

Create a small Groovy script that can be used in `settings.gradle`
files to extend `repositories` to support the various maven
repositories required for our build.

See gh-42333

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests-bootDistZipJarApp.gradle

@@ -15,8 +15,7 @@ repositories {
 		}
 	}
 	mavenCentral()
-	maven { url 'https://repo.spring.io/milestone' }
-	maven { url 'https://repo.spring.io/snapshot' }
+	spring.mavenRepositories()
 }
 
 dependencies {

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests-executableWarApp.gradle

@@ -15,8 +15,7 @@ repositories {
 		}
 	}
 	mavenCentral()
-	maven { url 'https://repo.spring.io/milestone' }
-	maven { url 'https://repo.spring.io/snapshot' }
+	spring.mavenRepositories()
 }
 
 dependencies {

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests-nativeApp.gradle

@@ -16,8 +16,7 @@ repositories {
 		}
 	}
 	mavenCentral()
-	maven { url 'https://repo.spring.io/milestone' }
-	maven { url 'https://repo.spring.io/snapshot' }
+	spring.mavenRepositories()
 }
 
 dependencies {

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests-plainDistZipJarApp.gradle

@@ -15,8 +15,7 @@ repositories {
 		}
 	}
 	mavenCentral()
-	maven { url 'https://repo.spring.io/milestone' }
-	maven { url 'https://repo.spring.io/snapshot' }
+	spring.mavenRepositories()
 }
 
 dependencies {

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests-plainWarApp.gradle

@@ -15,8 +15,7 @@ repositories {
 		}
 	}
 	mavenCentral()
-	maven { url 'https://repo.spring.io/milestone' }
-	maven { url 'https://repo.spring.io/snapshot' }
+	spring.mavenRepositories()
 }
 
 dependencies {

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests.gradle

@@ -14,8 +14,7 @@ repositories {
 		}
 	}
 	mavenCentral()
-	maven { url 'https://repo.spring.io/milestone' }
-	maven { url 'https://repo.spring.io/snapshot' }
+	spring.mavenRepositories()
 }
 
 dependencies {

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/settings.gradle

@@ -8,8 +8,7 @@ pluginManagement {
 				includeGroup "org.springframework.boot"
 			}
 		}
-		maven { url 'https://repo.spring.io/milestone' }
-		maven { url 'https://repo.spring.io/snapshot' }
+		spring.mavenRepositories()
 		gradlePluginPortal()
 	}
 	resolutionStrategy {