Fix generate_external_units_version_maturity() function in generate_units_documentation.py

What does this MR do and why?

We discussed with @amonl, to automatically generate a base release notes for Sylva-core which will probably included some info like, which external units we are using and their version, which we are maintaining etc...

for that we could rely on the existing generate_units_documentation.py script

This MR fix:

• a small typo in generate_units_documentation.py script to filter the external units
• add internal field to mgmt-cluster-ready unit
• add maturity "beta" to the units without maturity field

In a separate MR we could add more filter to only display the units maintains by Sylva teams etc...