[keyserver] Increase social_proof column of cookies table from varchar(255) to varchar(4096)

Summary:
Was experiencing issues populating the social_proof column of the cookies table because the string I was attempting to store exceeded the "width" of the column:

This diff just increases the size to ensure that the message fits.

Depends on D6134

Test Plan:

1. Ensure migration ran successfully:

2. Didn't test create-db, but confident things will work as expected.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6135