Disable image variant processing

[jazairi]

Why these changes are being introduced:

Active Storage attempts to load an image
processor gem as part of its initialization, even
if the app does not actively generate variants.
(ETD does not.)

Relevant ticket(s):

• ETD-668

How this addresses that need:

This explicitly disables the variant processor
to suppress the Active Storage initialization
error.

Side effects of this change:

If we ever do need image variants, we will need
the image_processing gem (or another image
processor).

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO

[coveralls]

coverage: 98.25%. remained the same — etd-668 into main

[JPrevost]

This feels way too easy.