release: Update package.xml version to 4.12.0

package.xml

 <?xml version='1.0'?>
  <package format='2'>
  <name>hpp-wholebody-step</name>
- <version>4.11.0</version>
+ <version>4.12.0</version>
  <description>Path planning for humanoid robot by sliding on the groud. </description>
 
  <maintainer email='hpp@laas.fr'>Joseph Mirabel</maintainer>