gddo-server/assets: bold redirect text

Text on the right side of the banner is bolded to emphasize
"Always use pkg.go.dev".

Screenshot:
https://photos.app.goo.gl/nNeRP8zpwi4jGuGk6

Change-Id: I97a7a6b10a5710810e9a9c652d8012492f3e4d95
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/275872
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Trust: Julie Qiu <julie@golang.org>