Board refactor - Split BoardColumn component [RUN AS-IF-FOSS]

What does this MR do?

Consolidate graphQLBoardLists feature flag by splitting BoardColumn and BoardHeader components to isolate boardStore and help with deprecation.

This is the first iteration of a process with the following goals:

• Reduce the number of mentions of graphQLBoardLists feature flag.
• Decouple boardsStore from the new version of boards.
• Increase clarity about which code is legacy and which code related to graphQL boards, so everyone can contribute more easily to boards and participate to its refactoring.
• Make the removing of the feature flag down the line easier.

Screenshots (strongly suggested)

No visual changes

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

Related to #273581 (closed)