Make workshop lead time be "at least 10 weeks"

[maneesha]

On the workshop request forms, we note that the necessary lead time for a workshop is "2-3 months." To be more precise, this should be changed to "at least 10 weeks." @danielle06 is working on making changes to Helpscout templates and other places we may have this language, so we should be sure to coordinate these changes to happen together.