[Change]: Migrate default MongoDB version from 6 to 7/8

[HeliosLHC]

Browsertrix Host

Self-Hosted

What change would you like to see?

The current default MongoDB image version is 6.0.5 mongo_image: "docker.io/library/mongo:6.0.5"

MongoDB 6.X.X will be EOL in July 31, 2025

Looking to see if MongoDB 7+ can be supported and set as the default version.

Additional details

No response