Replace deprecated Grafana SCSS styles

[ashharrison90]

Hi from the Grafana team 👋

In Grafana 12, we'll be removing Angular support. As part of that, we'll also be removing a lot of legacy CSS classes.
It looks like this plugin relies on a few of these classes:

• panel-empty

The CSS class names that Grafana provides can change at any time and should not be relied on as a stable external API. The recommendation would be to either:

• Use an existing component from the component library at @grafana/ui
• Use the theme object with emotion to style your components. The theme object is a stable API that is maintained according to SemVer.
• Apply these styles yourself within this repo. You can find the existing styles here as a reference.

Thanks again for your contributions and support! 🙌