Move Free tier heading for File Locking feature for added clarity

What does this MR do?

Our docs say File Locking is free.

This may be misleading because, as only 1/2 of file locking are available in our Free plan.

Exclusive file locks (using LFS) are free, whereas Default branch file and directory locks are Premium+.

This MR moves the Free tier indicator to the Exclusive File locks heading instead of the top level heading to help distinguish which File locking features are free.

Prompted by this conversation in Slack @lyle @izzyfee

Screenshots (strongly suggested)

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

@marcia @msedlakjakubowski for review! 🙏