doc/go1.14: disable text/template processing in HTML page

HTML pages served by the website have the option to opt-in to template
processing, by including "Template: true" in the page metadata.
This functionality is documented at
https://github.com/golang/tools/blob/403f1254bdfd3da27c92a0e9e37dd180a9a82b3c/godoc/template.go#L5-L30.

Historically, the Go 1 release notes have used template processing
to a great extent, but release notes for all subsequent major Go
releases have not.

Since this feature is generally not used and not very well known,
it tends to do more harm than good by making it possible for errors
in the template to prevent the release notes from showing up at all.

Disable this feature for Go 1.14 release notes and onwards.
We can consider enabling it when there's a stronger need for it.

Fixes #37072
Updates #37070

Change-Id: If93553d52df12544b46c4edcf3aa5eddc2a155ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/218058


Reviewed-by: Ian Lance Taylor <iant@golang.org>