[FF] rollout image_lightboxes

Remove the feature flag image_lightboxes to enable widespread adoption.

Feature flag is currently in use to test for bugs given the broad use across GLFM surfaces as well as the potential for many different image formats.

Feature flag was available as of October 3, 2025 (MR), intended testing period is 2 weeks, if no bugs are identified after 1 week this should be shortened; if many bugs are found testing period will be extended until no bugs remain.