Fix release train version's comparison with other version types

Fixes gh-24782

buildSrc/src/main/java/org/springframework/boot/build/bom/bomr/version/ReleaseTrainDependencyVersion.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -48,7 +48,7 @@ final class ReleaseTrainDependencyVersion implements DependencyVersion {
 	@Override
 	public int compareTo(DependencyVersion other) {
 		if (!(other instanceof ReleaseTrainDependencyVersion)) {
-			return 0;
+			return -1;
 		}
 		ReleaseTrainDependencyVersion otherReleaseTrain = (ReleaseTrainDependencyVersion) other;
 		int comparison = this.releaseTrain.compareTo(otherReleaseTrain.releaseTrain);

buildSrc/src/test/java/org/springframework/boot/build/bom/bomr/version/ReleaseTrainDependencyVersionTests.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -92,6 +92,14 @@ public class ReleaseTrainDependencyVersionTests {
 		assertThat(version("Kay-SR6").isSameMinorAndNewerThan(version("Kay-SR7"))).isFalse();
 	}
 
+	@Test
+	void whenComparedWithADifferentDependencyVersionTypeThenTheResultsAreNonZero() {
+		ReleaseTrainDependencyVersion dysprosium = ReleaseTrainDependencyVersion.parse("Dysprosium-SR16");
+		ArtifactVersionDependencyVersion twentyTwenty = ArtifactVersionDependencyVersion.parse("2020.0.0");
+		assertThat(dysprosium.compareTo(twentyTwenty)).isLessThan(0);
+		assertThat(twentyTwenty.compareTo(dysprosium)).isGreaterThan(0);
+	}
+
 	private static ReleaseTrainDependencyVersion version(String input) {
 		return ReleaseTrainDependencyVersion.parse(input);
 	}