Use grafana/ui components in examples rather than referring to gf class names

[marefr]

Use grafana/ui components in examples rather than referring to gf class names. Below an example that are using gf-classes and should be able to use forms components: https://github.com/grafana/grafana-plugin-examples/blob/a57c63cf9211df73b66bb0eb5f937b39262ffded/examples/datasource-http-backend/src/components/ConfigEditor.tsx#L14-L22

[sympatheticmoose]

Couple of potential follow ups:

• Create a new check within the validator to warn on using gf class names
• Provide more documentation for frontend development to advertise the use of the appropriate components for common elements