application/javascript is being deprecated per
https://datatracker.ietf.org/doc/draft-ietf-dispatch-javascript-mjs/

Specify a charset to be consistent with other text/* mime types.

Fixes #32351

Change-Id: I7300f6cfdbcf574103764991cb75172a252a3400
Reviewed-on: https://go-review.googlesource.com/c/go/+/186927
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>