Fix teacher application export via smaller batch size for Google Sheet uploads

[bencodeorg]

Our export of teacher application data to Google Sheets is failing due to a connection timeout (ie, the upload into the gsheet takes too long). We faced this issue last year, and introduced batching to resolve it. This PR reduces the batch size even further.

Testing story

Using some instructions here, I created 4000 dummy applications locally. I was able to reproduce the error before changing the batch size. Then, I successfully uploaded those applications to a test gsheet in a little over 20 minutes.

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• 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