content/static/html: gate deprecated/retracted by experiment
content/static/html: gate deprecated/retracted by experiment

For golang/go#43265
For golang/go#41321

Change-Id: Ie0296d9a00c188621029252f6f91109860a1a0bc
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/297529
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite