Rename .gitlab_pages_shared_secret to .gitlab_pages_secret

What does this MR do?

After the cleanup done in omnibus-gitlab!3669 (merged), gitlab-pages!193 (merged), and !18172 (merged) in omnibus-gitlab!3705 (merged) we used .gitlab_pages_secret as filename instead of .gitlab_pages_shared_secret (introduced in gitlab-foss!32732 (merged)).

This MR updates the Rails app to use .gitlab_pages_secret as default filename. The API endpoint that needs this file is behind feature flag and not used yet.

Screenshots

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

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