Use YAML compatible classpath.idx format

Update the `classpath.idx` format to align with `layers.idx` and allow
third-parties can parse it as YAML

Closes gh-20861

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/AbstractExecutableArchiveLauncherTests.java

@@ -68,9 +68,9 @@ public abstract class AbstractExecutableArchiveLauncherTests {
 			JarEntry indexEntry = new JarEntry(entryPrefix + "/classpath.idx");
 			jarOutputStream.putNextEntry(indexEntry);
 			Writer writer = new OutputStreamWriter(jarOutputStream, StandardCharsets.UTF_8);
-			writer.write("BOOT-INF/lib/foo.jar\n");
-			writer.write("BOOT-INF/lib/bar.jar\n");
-			writer.write("BOOT-INF/lib/baz.jar\n");
+			writer.write("- \"BOOT-INF/lib/foo.jar\"\n");
+			writer.write("- \"BOOT-INF/lib/bar.jar\"\n");
+			writer.write("- \"BOOT-INF/lib/baz.jar\"\n");
 			writer.flush();
 		}
 		addNestedJars(entryPrefix, "/lib/foo.jar", jarOutputStream);

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/resources/org/springframework/boot/loader/classpath-index-file.idx

@@ -1,5 +1,5 @@
-a.jar
-b.jar
-c.jar
-d.jar
-e.jar
+- "a.jar"
+- "b.jar"
+- "c.jar"
+- "d.jar"
+- "e.jar"