chore: move to channel 6/stable in mongodb

[patriciareinoso]

Description

This PR sets the the 6/stable channel for mongodb charm instead of 6/beta
canonical/mongodb-k8s-operator#218 seems fixed in 6/stable so the tests can be restored

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library