Remove java.level property from examples

The `java.level` property is deprecated and will be removed from a
future plugin parent pom.

See jenkinsci/plugin-pom#522 for the plugin pom
deprecation of `java.level`.

See the following additional references:

* jenkinsci/plugin-pom#478 (comment)
* jenkinsci/plugin-pom#478 (comment)
* jenkinsci/plugin-pom#522 (comment)

Also uses newer Jenkins version in the example.  Would have liked to use
the automatic replacement as is done in the "Choosing a Jenkins version"
baseline page, but that was a larger change than I'm ready to make in
this pull request.

content/doc/developer/persistence/incompatible-releases.adoc

@@ -18,8 +18,7 @@ it is possible to set the `hpi.compatibleSinceVersion` property to define the ol
 [source,xml]
 ----
 <properties>
-  <jenkins.version>2.60.3</jenkins.version>
-  <java.level>8</java.level>
+  <jenkins.version>2.289.3</jenkins.version>
   <hpi.compatibleSinceVersion>1.0</hpi.compatibleSinceVersion>
 </properties>
 ----

content/doc/developer/plugin-development/mark-a-plugin-incompatible.adoc

@@ -16,8 +16,7 @@ Starting from Plugin POM 3.33, it is possible to set a `hpi.compatibleSinceVersi
 [source,xml]
 ----
 <properties>
-    <jenkins.version>2.60.3</jenkins.version>
-    <java.level>8</java.level>
+    <jenkins.version>2.289.3</jenkins.version>
     <hpi.compatibleSinceVersion>1.0</hpi.compatibleSinceVersion>
 </properties>
 ----