feat: enable Etag caching

What does this MR do?

We've recently introduced and enabled Etag caching. This MR removes the feature flag. I originally wanted to add it just to make the rollout more controlled.

Closes #672 (closed)

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines