Plugin update PRs pile up and users install old plugins #202

Summary

Users who installed a plugin got an old version: the catalog served alb v0.32.3 while v0.32.31 was out. Each release and the daily scan opened a new update PR and never reused an open one, so 65 piled up unmerged.

I pinned the release workflows to the shared action that reuses one PR per plugin, and made the scan reuse it too. I closed the 62 superseded PRs.

Acceptance criteria

• The four update PRs merged, Update dns plugin to v0.10.5 #194 (dns), Update network plugin to v0.32.31 #195 (network), Update alb plugin to v0.32.31 #196 (alb) and Update the search plugin to v0.8.4 #200 (search), and the index serves each plugin's newest release (chore: regenerate index.yaml #201)
• compute's release updated its open PR instead of opening one (v0.12.11)
• The daily scan reuses the plugin's open PR instead of opening another
• The next network-services-operator and dns-operator releases leave one open update PR per plugin

Related to datum-cloud/actions#98, datum-cloud/network-services-operator#668, datum-cloud/dns-operator#256, datum-cloud/compute#495 and #199