internal/frontend: move deprecation info to VersionList
internal/frontend: move deprecation info to VersionList

Move the deprecation information from VersionListKey to VersionList.
The former is intended as a map key, not a container for arbitrary
major-version data.

For golang/go#48223

Change-Id: Ifcbd72f368b68d627cb98ee4afa93ab6e3b81d17
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/348109
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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