Mitigate rate limit issues by utilizing `mirror.gcr.io` as default DB repository

[knqyf263]

Description

The Trivy team has been exploring solutions to address the rate limit issues encountered during database updates and has provided some workarounds. It has come to our attention that mirror.gcr.io functions as a mirror for Docker Hub without imposing rate limits. By configuring mirror.gcr.io as the default repository for the Trivy databases, we can potentially resolve the current rate limit challenges. Additionally, incorporating GHCR as a secondary repository could provide further redundancy and reliability.

For reference, please see the related discussion

[DmitriyLewen]

Added in #7679