Migrate deprecated singlestat panels to stat panels in Grafana dashboard

[coderabbitai]

Issue Description

The JVM metrics dashboard () currently uses deprecated singlestat panels, which were replaced by stat panels in Grafana 8+.

Problem

• singlestat panels are deprecated in Grafana ≥ 8
• While they still render in Grafana 12, they are flagged as legacy
• Future major versions of Grafana will drop support for singlestat panels

Solution

Migrate all singlestat panel types to stat panel types in the dashboard JSON file to ensure forward compatibility.

Tasks

• Identify all panels with "type": "singlestat" in the dashboard
• Change panel type from singlestat to stat
• Verify that all properties and configurations are compatible with the stat panel type
• Adjust configurations if necessary for proper rendering
• Test the updated dashboard with current Grafana version

References

• PR: Feature/spring prometheus grafana #33
• Comment: Feature/spring prometheus grafana #33 (comment)
• Requested by: @igorcampos-dev