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

[vbotbuildovich]

Backport #29824 to branch v25.3.x. This issue was created as a placeholder because the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v25.3.x
git checkout -b manual-backport-29824-v25.3.x-902
git cherry-pick -x 1ea6e4e51e c77c86edde

git push origin manual-backport-29824-v25.3.x-902
gh pr create \
  --title "[v25.3.x] cluster/feature_manager: use insert_or_assign for version updates" \
  --base "v25.3.x" \
  --label "kind/backport" \
  --head "manual-backport-29824-v25.3.x-902" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v25.3.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/29824 "