databases/mongodb70: add flavor for RPI-compatible version
The default flavor is the upstream supported version. For older
ARM CPUs we have an -armv80a flavor now. MongoDB is known to be used by
net-mgmt/unifi* users on this hardware. MongoDB 6.0 is planned to go out
of support in July 2025 [1], so this is preparation to make MongoDB 7.0
the default dependency for Unifi.