Narrow criteria for hidden property paths

A previous change intended to prevent temporary directories created as
an implementation detail of mounting volumes on kubernetes pods from
being used as a source for property files had a side-effect of also
preventing other types of hidden directories from being recognized. This
commit narrows the criteria for considering a directory as hidden,
using the kubernetes `..` prefix convention instead of the Unix `.`
prefix.

Fixes gh-23983

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/config/ConfigFileApplicationListener.java

@@ -569,7 +569,7 @@ public class ConfigFileApplicationListener implements EnvironmentPostProcessor,
 		private boolean hasHiddenPathElement(Resource resource) throws IOException {
 			String cleanPath = StringUtils.cleanPath(resource.getFile().getAbsolutePath());
 			for (Path value : Paths.get(cleanPath)) {
-				if (value.toString().startsWith(".")) {
+				if (value.toString().startsWith("..")) {
 					return true;
 				}
 			}