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/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"