Update the default version to 5.6.0-RC3

5 jobs for release
Status Name Job ID Coverage
  External
canceled paraview-bigmac-osx-shared-release+osx10.8+superbuild

canceled paraview-hera-windows-shared-release+nompi+superbuild

canceled paraview-hera-windows-shared-release+superbuild

canceled paraview-miranda-windows-shared-release+superbuild

canceled paraview-pvbinsdash-linux-shared-release+superbuild