Enable Apollo Boards by default

What does this MR do and why?

Following is a quick summary of what happened, and why this MR is needed;

• As a part of &9026 (closed), we refactored Boards app to move away from Vuex and use GraphQL exclusively, this has no user-facing impact and was done mainly for maintenance and extensibility of Boards app going forward.
• This refactor was done behind the feature flag :apollo_boards.
• We were supposed to enable :apollo_boards by default for self-managed users by updating config/feature_flags/development/apollo_boards.yml in %16.7 but we missed it.
• We did code-cleanup in %16.7 as a part of !137614 (merged) & !138006 (merged) assuming that flag was enabled already.
• Self-managed instances which upgraded to 16.7 ran into a situation where the feature flag was disabled but the legacy implementation of boards simply didn't exist and that broke boards completely for these users.
• Only workaround for these users is to either manually enable the feature flag, or wait till %16.8 is release where we have enabled the FF via !140845 (merged).

This MR mitigates this by enabling the :apollo_boards feature flag to true by default so instances running %16.7 can upgrade to a patch without waiting for 16.8.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).

Related to #381210 (closed)