release: Update package.xml version to 3.12.0

package.xml

 <?xml version="1.0"?>
 <package format="3">
   <name>example-robot-data</name>
-  <version>3.11.0</version>
+  <version>3.12.0</version>
   <description>Set of robot URDFs for benchmarking and developed examples.</description>
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   Please check the repository URL for project maintainer. -->