assets: update banner text and font size

Banner text size is increased to 18px.

The text is updated to indicate that redirects will happen in the next
few weeks.

Learn more now points to https://golang.org/issue/43178.

Change-Id: I889cee54acb5282c3b9d2c80d840074b1278ef00
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285254
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
2 files changed