fix(plugin): temporarily watch for old Plugin and new PluginDefinition labels

[IvoGoman]

Description

Watching on both labels is required, as the deprecated greenhouse.sap/plugin will only be removed after reconciliation.

What type of PR is this? (check all applicable)

• [ ] 🍕 Feature
• [x] 🐛 Bug Fix
• [ ] 📝 Documentation Update
• [ ] 🎨 Style
• [ ] 🧑‍💻 Code Refactor
• [ ] 🔥 Performance Improvements
• [ ] ✅ Test
• [ ] 🤖 Build
• [ ] 🔁 CI
• [ ] 📦 Chore (Release)
• [ ] ⏩ Revert

Related Tickets & Documents

• Related Issue # (issue)
• Closes # (issue)
• Fixes # (issue)

Remove if not applicable

Added tests?

• [ ] 👍 yes
• [x] 🙅 no, because they aren't needed
• [ ] 🙋 no, because I need help
• [ ] Separate ticket for tests # (issue/pr)

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Added to documentation?

• [ ] 📜 README.md
• [ ] 🤝 Documentation pages updated
• [x] 🙅 no documentation needed
• [ ] (if applicable) generated OpenAPI docs for CRD changes

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes