[backend] Remove the 'securityContext' field based on the KFP version

[jesuino]

Environment

• Kale version:
  2.x - upstream

Steps to reproduce

• Use Kale against KFP 2.15.0 or earlier versions
• Users can't use it because these versions to do support the new securityContext field, leading to errors

Expected result

Being able to use Kale with older 2.x KFP versions

Materials and Reference

The 'securityContext' field could be removed from the generated DSL when using older KFP versions.

[ada333]

/assign

[ada333]

/assign @jesuino