Alerts for Application Gateway References, SQL Managed Instances and Sql Server Databases

[sihbher]

Overview/Summary

Replace this with a brief description of what this Pull Request fixes, changes, etc.
I added alerts for Application Gateway References, SQL Managed Instances and Sql Server Databases

This PR fixes/adds/changes/removes

1. References added for Application Gateway
2. Alerts added for SQL Managed Instances
3. Alerts added for SQL Server Databases

As part of this Pull Request I have

• Read the Contribution Guide and ensured this PR is compliant with the guide
• Checked for duplicate Pull Requests
• Associated it with relevant GitHub Issues or ADO Work Items (Internal Only)
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Ensured PR tests are passing
• Updated relevant and associated documentation (e.g. Contribution Guide, Docs etc.)

I missed the linting errors in my first review

[JoeyBarnes]

Please fix the YAML linting errors. The warnings can be ignored for now.

https://github.com/Azure/azure-monitor-baseline-alerts/actions/runs/7134476286/job/19429380666?pr=88#step:4:95