internal/database: change log to debug level
internal/database: change log to debug level

Since serialization failures are not real errors, downgrade these from
warning to debug to reduce noise in the seeddb logs.

Change-Id: I800e2a1588d57156b0ebe0f39c7c7ef677f3583b
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/338593
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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