Simplify mode installation for Elemental content variations

[mfendeksilverstripe]

Description

Currently, the module requires a template override to be added to enable the Elemental content variations. This can be changed to an extension point after silverstripe/silverstripe-elemental#1321 is completed.

Additional context or points of discussion

Extension point is available in Elemental 6.

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)