Update tests to avoid deprecation warnings with Gradle 8.12

[wilkinsona]

Running spring-boot-gradle-plugin's tests with Gradle 8.12-rc-1 results in a number of deprecation warnings that cause the tests to fail. For example:

Space-assignment syntax in Groovy DSL has been deprecated. This is scheduled to be removed in Gradle 10.0. Use assignment ('url = <value>') instead. Consult the upgrading guide for further information: https://docs.gradle.org/8.12-rc-1/userguide/upgrading_version_8.html#groovy_space_assignment_syntax	
	at org.gradle.internal.instantiation.generator.AsmBackedClassGenerator.logGroovySpaceAssignmentDeprecation(AsmBackedClassGenerator.java:262)	
	at org.gradle.api.internal.artifacts.repositories.DefaultMavenArtifactRepository_Decorated.url(Unknown Source)	
	at java.base/java.lang.reflect.Method.invoke(Method.java:569)	
	at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:323)	
	at org.gradle.internal.extensibility.MixInClosurePropertiesAsMethodsDynamicObject.tryInvokeMethod(MixInClosurePropertiesAsMethodsDynamicObject.java:38)	
	at build_8dj7u9xsyqvh3vf3vgxwhmcpe$_run_closure2$_closure12.doCall$original(/private/var/folders/2w/mhq2nt4d0xx2w6w4c36n468h0000gn/T/gradle-4424519662978718210/build.gradle:30)

This particular warning in caused by this:

spring-boot/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests-customLayers.gradle

Line 30 in ccc1b5d

maven { url 'repository' }

There are several others.

We should update the tests to avoid these warnings, ideally in a way that retains compatibility with Gradle 7.6.x to avoid having to have the tests use different build scripts for different versions of Gradle.