fix: Fixing the fiveg_nrf endpoint name and pinning Juju version to 3.3.0.0

[gatici]

Description

• fiveg_nrf endpoint name was different than the one in the metadata.yaml. This PR puts the correct name.
• Pinning juju version to 3.3.0.0, which is last working. With 3.3.1.0 tests started failing because of

 File "/home/ubuntu/github-runner/_work/sdcore-tests/sdcore-tests/.tox/integration/lib/python3.10/site-packages/juju/version.py", line 19, in <module>
    CLIENT_VERSION = re.search(r'\d+\.\d+\.\d+', open(VERSION_FILE_PATH).read().strip()).group()
FileNotFoundError: [Errno 2] No such file or directory: '/home/ubuntu/github-runner/_work/sdcore-tests/sdcore-tests/.tox/integration/lib/python3.10/site-packages/VERSION'

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