Set `has_external_issue_tracker` to `DEFAULT FALSE` and `NOT NULL`

What does this MR do?

This change contains migrations for projects.has_external_issue_tracker to become a NOT NULL column that defaults to FALSE. It is similar to !50916 (merged).

This is possible after the PG Trigger added in !51852 (merged) that always maintains projects.has_external_issue_tracker as a boolean and never a NULL.

It includes a data migration to set all historical NULL columns to be either TRUE or FALSE.

The data migration also fixes the historical bad data for has_external_issue_tracker #273574 (closed).

The issue for the validation of the NOT NULL constraint in %13.10 is #296719 (closed).

Related issues: #273574 (closed) #290715 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #290715 (closed)