Restore IMAGE_FILTER_FLAGS default to label filter (backport to 19-2-stable, closes #39621)

What does this MR do?

Backport of commit da8737b0 (MR !6985 (merged)) to 19-2-stable.

Restores IMAGE_FILTER_FLAGS default from empty to label=com.gitlab.gitlab-runner.managed=true, matching the pre-!6912 behaviour where only runner-managed images were pruned.

Why was this MR needed?

MR !6912 (merged) introduced a regression: IMAGE_FILTER_FLAGS defaulted to empty, causing docker image prune -af to delete all unused images on the host during apt upgrade postinst. This was reported in #39621 (closed) and by users @jiegec and @hudakharrufa.

The fix was merged to main via MR !6985 (merged) but was never backported to 19-2-stable, leaving the regression present in the 19.2.x package line.

What are the relevant issue numbers?

Related to !6912 (merged), !6985 (merged), #39621 (closed)