Narrow down the target type of logging.level

While `logging.level` is managed in a dedicated way in
`LoggingApplicationListener`, the value of the map are String values
representing the level to apply. The meta-data now exposes that type
instead of raw `java.lang.Object`

Closes gh-2388

spring-boot/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -37,7 +37,7 @@
   },
   {
     "name": "logging.level",
-    "type": "java.util.Map<java.lang.String,java.lang.Object>",
+    "type": "java.util.Map<java.lang.String,java.lang.String>",
     "description": "Log levels severity mapping. Use 'root' for the root logger.",
     "sourceType": "org.springframework.boot.logging.LoggingApplicationListener"
   },