Remove container_registry_display_supported_platforms feature flag

What does this MR do and why?

Cleans up the container_registry_display_supported_platforms feature flag.

Feature flag was rolled out in 19.1.

References

• Rollout issue: #600202 (closed)
• Feature issue: #369852 (closed)
• Introduced by: !228384 (merged)

Screenshots or screen recordings

See validation steps

How to set up and validate locally

Testing I have done to verify feature still works:

1. Log in

echo "$PAT" | docker login registry.test:5100 -u root --password-stdin

2. Grab two real single-arch variant digest.

REPO=registry.test:5100/<group>/<project>
AMD64=$(docker manifest inspect alpine:3.20 | jq -r '.manifests[] | select(.platform.os=="linux" and .platform.architecture=="amd64").digest')
ARM64=$(docker manifest inspect alpine:3.20 | jq -r '.manifests[] | select(.platform.os=="linux" and .platform.architecture=="arm64").digest')
docker pull alpine@${AMD64} && docker:amd64 && docker push ${REPO}:amd64
docker pull alpine@${ARM64} && docker tag alpine@${ARM64} ${REPO}:arm64 && docker push ${REPO}:arm64

3. Stitch them into a manifest list = the multi-arch index.

docker manifest create --insecure ${REPO}:multiarch ${REPO}:amd64 ${REPO}:arm64
docker manifest push   --insecure ${REPO}:multiarch

4. Confirm it's an index with two platforms

docker manifest inspect --insecure ${REPO}:multiarch | jq -r '.mediaType, (.manifests[] | "\(.platform.os)/\(.platform.architecture)")'

Verify in the UI

1. Go to the project → Deploy → Container registry
2. Expand the multiarch tag → Show details.

• Expect a Supported platforms row rm64/v8 badges.

3. Expand a single-arch tag (amd64/arm64) → no platforms row (it's a plain image manifest, not an index).

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.