Deprecate supporting null scheduling_type of ci_builds

What does this MR do?

From #205426

When implementing empty needs, we tried to make it backward compatible. It means we considered existing ci_builds without scheduling_type, and implemented populating those on the fly. That's because our code always needs to ensure scheduling_type existence of processables.

This MR will:

• deprecate handling null scheduling_type of ci_builds and hide behind a FF.
• Add background migration that populates null scheduling_types

After this, we can drop this completely.

Question

Now we have a background migration that needs to be completed until 13.0.

Is it correct workflow to remove an on-the-fly migration code?

Screenshots

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

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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