Add datasource variable and fix deprecated panels on Grafana dashboard

[jonatasbaldin]

Hi there 👋

This PR adds a datasource variable to the Grafana dashboard, so when importing, it will ask which instance to pick (and pick the default one when importing with automated processes).

Also, it upgrades the deprecated Angular panels into the Time Series panels, as shown on the image below:

Do you follow the guidelines?

• [x] I have tested my changes
• [x] There is no breaking changes
• [x] I really tested my changes and there is no regression
• [x] Ideally, my commit messages follow the Conventional Commits specification
• [x] I read this document: https://miniflux.app/faq.html#pull-request