Tweak "Schedule a new pipeline" page cron syntax link

What does this MR do and why?

This MR:

• updates the Cron syntax link to be a Learn more. link to https://docs.gitlab.com/ee/topics/cron/ as proposed by @marcel.amirault.
• updates all the relevant locale files (I think!) to reflect this change
• updates tests relevant to this part of the frontend

Screenshots or screen recordings

New Learn more. link:

Bottom left shows the new link destination:

How to set up and validate locally

These are the steps I used to verify that the change worked as intended:

1. Pulled MR branch down locally into the gitlab fork managed by my local gitlab-development-kit
2. Ran gdk start
3. Took screenshots (seen above) on http://localhost:3000/flightjs/Flight/-/pipeline_schedules/new

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #341667 (closed)