internal/middleware: log health checks at debug level
internal/middleware: log health checks at debug level

/healthz requests clutter the log. Downgrade them to debug level.

Change-Id: Id6124ec3cc9307017d1bc1050c201bae0da70ac5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/254377
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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