Change cut_off_date to not be required for paid feature

The following discussion from !146 (merged) should be addressed:

• @mkaeppler started a discussion: (+2 comments)

  question: Should this have a cut-off-date defined too? Otherwise the feature will be considered accessible even without these add-ons:

  cut_off_date: 2024-02-15T00:00:00+00:00

  (this was the launch date for Duo Pro)

  @nmilojevic1 could you keep me honest on this one? It's a little odd at this point, now that launching Pro and Enterprise is in the past, to keep defining these cut-off dates. Perhaps we should consider flipping this around so that by default add_ons defined implies you must own these, and only pre-launch of such an add-on we define the cut-off-date?

We should change this to:

• if add_ons is empty, the feature does not require add_ons
• if add_ons is configured and cut_off_date is defined and set in the future, the feature does not require add_ons until the cut_off_date is in the past
• if add_ons is configured and cut_off_date is not defined, or cut_off_date is in the past, the add_on is required.
• we still need to figure out if cut_off_date applies to license_types as well

Since cut_off_date is just a datum, this also requires changes in gitlab-rails where this field is interpreted.