[v25.3.x] cluster/feature_manager: use insert_or_assign for version updates#29880

pgellert merged 1 commit intoredpanda-data:v25.3.xredpanda-data/redpanda:v25.3.xfrom

pgellert:manual-backport-29824-v25.3.x-902pgellert/redpanda:manual-backport-29824-v25.3.x-902Copy head branch name to clipboard