Use shorter text for "security training promo to MR widget"

Current	Proposal
Do not show again	Don't show again

Advantages ✅

• Allow us to reuse an already created localization > https://gitlab.com/gitlab-org/gitlab/-/blob/master/locale/gitlab.pot#L14162.
• Due to the small real estate of the MR widget, it might be better to go with a shorter text, so everything fits better in smaller viewports.

Background information

The following discussion from !99547 (merged) should be addressed:

• @dftian started a discussion: (+3 comments)

  We already have a localization for Don't show again. Can we re-use that, or do we want a separate one for Do not show again?

• Ah I see it now! Do not show again vs Don't show again 🤓 I do favor the latter shorter one; since this is still behind a feature flag, and we're nearing the end of the sprint, I'll just match the mock for now. I'll capture this suggestion in a follow-up issue and bring it up with Becka 😀