REF - Fix Campaign index names, add default value for created_date

[colemanw]

Overview

Fix misnamed indexes in the civicrm_campaign table and add default value for created_date field.
This is a bite-sized chunk of #25998

Before

Indexes incorrectly named. Our convention is to prefix unique indexes with UI_ and non-unique ones with index_. Since these indexes are not unique, they had the wrong prefix.

After

Fixed indexes, added a default value for created_date field instead of supplying it with PHP.

[civibot]

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[aydun]

Looks reasonable - and rebuilding those indexes is not likely to be a big hit during upgrade.