Update Linux containerization to use STK Engine 12.6 as default

[jorgepiloto]

💡 Suggestion
With the latest release of STK 12.6, artifacts for STK Engine have been generated. Thus, it is possible to update references in the linux containerization directory to this new release, that is replacing 12.4 to 12.6.

In fact, this update could be expanded to other parts of the repository where references to a version are made.

It may be worth to explore some automation using GitHub actions for solving this task now and for future releases too.