internal/play: add goprevplay, load versions from back ends
internal/play: add goprevplay, load versions from back ends

This will let us have "Go 1.17" in the dropdown on go.dev/play.
And when the next release happens and goprevplay and play are
updated, the versions will update automatically in the UI.

Change-Id: Id76f89767c3a91a2c9e27c9fbdad5d6fd9674ab2
Reviewed-on: https://go-review.googlesource.com/c/website/+/393840
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website