internal/frontend: use new pageTypes in latestMinorVersion
internal/frontend: use new pageTypes in latestMinorVersion

Instead of use legacyPageTypeModule and legacyPageTypeModuleStd in
latestMinorVersion, use pageTypeModule and pageTypeModuleStd. These
consts map to the same strings, but the former set will be deleted soon.

Change-Id: I3abb6b3d08992977939cc4c5e76f84a7b1146a63
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/270237
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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