Change default raw value of ci-variables tables

What does this MR do and why?

❗ TODO: move this to post_migrations ❗

This is the first step of the problem stated in #374616 (closed).

Step	MR
1. Change the default value of raw to false.	<-- HERE
2. Add background migration to alter old columns. We can do this safely because we never use raw now.	TODO
3. (Optional) Change the logic of to_runner_variable to support raw.	TODO

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