Deprecate unused ErrorController interface method

This commit marks as deprecated an interface method that is no longer
used, and changes all internal implementations to return `null` to
make the fact that the return value is not used more obvious.

Fixes gh-19844

Author: scottfrederick

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/error/BasicErrorController.java

@@ -83,7 +83,7 @@ public BasicErrorController(ErrorAttributes errorAttributes, ErrorProperties err
 
 	@Override
 	public String getErrorPath() {
-		return this.errorProperties.getPath();
+		return null;
 	}
 
 	@RequestMapping(produces = MediaType.TEXT_HTML_VALUE)

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/error/ErrorController.java

@@ -19,19 +19,23 @@
 import org.springframework.stereotype.Controller;
 
 /**
- * Marker interface used to indicate that a {@link Controller @Controller} is used to
- * render errors. Primarily used to know the error paths that will not need to be secured.
+ * Marker interface used to identify a {@link Controller @Controller} that should be used
+ * to render errors.
  *
  * @author Phillip Webb
+ * @author Scott Frederick
  * @since 2.0.0
  */
 @FunctionalInterface
 public interface ErrorController {
 
 	/**
-	 * Returns the path of the error page.
+	 * The return value from this method is not used; the property `server.error.path`
+	 * must be set to override the default error page path.
 	 * @return the error path
+	 * @deprecated since 2.3.0 in favor of setting the property `server.error.path`
 	 */
+	@Deprecated
 	String getErrorPath();
 
 }