Return summer workshops in order of start date

[bencodeorg]

Description

Updates list of summer workshops shown to teachers using the "search for your regional partner by zip" page to sort by the start date of the workshop, not the date the workshop was created. There was already a method that does exactly what I wanted, so nothing new!

Testing story

Tested locally. Also looked to see if there's anywhere else that we call upcoming_summer_workshops and didn't find anything -- that said, a search for upcoming_summer_workshops didn't return its use in the serializer that is responsible for its behavior on this page, which I don't understand.

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[islemaster]

Great find!