Backfill first-commit dates of repo's in projects.json and update validation

[schneedotdev]

Summary

Within projects.json, submissions with a category of "repo" are missing date information, useful for internal metrics. Add dates for the first commit of each repo and update date validation for json entries.

Acceptance Criteria

• All objects that include category: "repo" should include a date
• Update validate.go to address date format for all objects

Breadcrumbs/Resources

N/A

Obstacles

N/A