Bulk generate coupons using capital letters only

Bulk coupon code generation currently uses both uppercase and lowercase letters even though coupon code matching is case-insensitive due to default MySQL behavior. This means two coupon codes could be generated that would appear to be different even though either one might match on load (e.g., abcd and ABCD).

While we could change the database to use case-sensitive matching, this would not be backwards compatible. Let's instead update the CouponCodeGenerator to remove lowercase letters from generated patterns.