Backfill environment_id on deployment_merge_requests

What does this MR do?

This MR includes migrations to backfill environment_id on deployment_merge_requests table.

environment_id is nullable and was introduced to deal with duplicated environments in MR deployment tracking #199256 (closed)

Here we backfill the old entries that still have a NULL environment_id

We batch entries with ranges of merge_request_id and schedule some background migrations.

Related to #208905

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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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
• Security reports checked/validated by a reviewer from the AppSec team