Give code blocks a real language instead of unknown (4.x)

src/site/markdown/examples/attached-jar.md.vm

@@ -33,7 +33,7 @@ Add a `classifier` as well.
 Otherwise, it replaces the default use of the JAR Plugin. It does not add a second artifact.
 The `classifier` is also required to create more than one artifact.
 
-```unknown
+```xml
 <project>
   ...
   <build>

src/site/markdown/examples/include-exclude.md.vm

@@ -29,7 +29,7 @@ under the License.
 Add `<includes>`/`<include>` or `<excludes>`/`<exclude>` in your `pom.xml`.
 These elements specify the fileset patterns to include or exclude.
 
-```unknown
+```xml
 <project>
   ...
   <build>

src/site/markdown/examples/manifest-customization.md.vm

@@ -42,7 +42,7 @@ You can alter the default manifest with the `archive` configuration element.
 The example below shows some of the available configuration options.
 For more information, see the [Maven Archiver reference](/shared/maven-archiver/index.html).
 
-```unknown
+```xml
 <project>
   ...
   <build>