Disable generation of Gradle metadata for spring-boot-dependencies

Gradle's metadata for a platform does not provide a way to define
exclusions in the same way that can be done in <dependencyManagement>
in a Mavne bom. This means that Gradle 6 users lose the exclusions
that are defined in the pom as Gradle 6 will use the metadata rather
than the pom.

Until such a time as Gradle's metadata supports the configuration of
such exclusions we can avoid the problem by disabling the metadata's
publication.

Fixes gh-21350

spring-boot-project/spring-boot-dependencies/build.gradle

@@ -1847,3 +1847,7 @@ bom {
 		}
 	}
 }
+
+generateMetadataFileForMavenPublication {
+	enabled = false
+}