Fix double-counted savings plan purchases

[flanakin]

🛠️ Description

Change EffectiveCost for savings plan purchases to 0 to work around a bug in Cost Management where FOCUS data is inconsistent with the Azure portal.

Fixes #1092

📋 Checklist

🔬 How did you test this change?

• [ ] 🤏 Lint tests
• [ ] 🤞 PS -WhatIf / az validate
• [ ] 👍 Manually deployed + verified
• [ ] 💪 Unit tests
• [ ] 🙌 Integration tests

🙋‍♀️ Do any of the following that apply?

• [ ] 🚨 This is a breaking change.
• [x] 🤏 The change is less than 20 lines of code.

📑 Did you update docs/changelog.md?

• [x] ✅ Updated changelog (required for dev PRs)
• [ ] ➡️ Will add log in a future PR (feature branch PRs only)
• [ ] ❎ Log not needed (small/internal change)

📖 Did you update documentation?

• [ ] ✅ Public docs in docs (required for dev)
• [ ] ✅ Internal dev docs in src (required for dev)
• [ ] ➡️ Will add docs in a future PR (feature branch PRs only)
• [x] ❎ Docs not needed (small/internal change)