fix: force inject a charset of UTF-8 into output css

[hoorayimhelping]

Helps with https://github.com/influxdata/ui/issues/4506

Changes

Force prepend @charset "UTF-8"; into the css file that is generated from our sass files during building.

Checklist

Check all that apply

• [x] Updated documentation to reflect changes
• [x] Added entry to top of Changelog with link to PR (not issue)
• [x] Tests pass
• [ ] Peer reviewed and approved