Allow feature flag names to be a symbol

What does this MR do?

Allow using symbols as feature flag key.

Why was this MR needed?

I made this mistake on https://gitlab.com/gitlab-org/gitlab-ee/issues/6964. We also have some mentions on our documentation that uses a symbol as the feature flag key, e.g., the Gitaly documentation mentions:

Feature.enable(:gitaly_feature_name)
Feature.disable(:gitaly_feature_name)

That's clearly wrong, and we should prevent people from making the same mistake. :)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6964