Backport of "Hide secrets manager settings behind feature flag instead of just the license" to 18.3

What does this MR do and why?

This is a bug fix to make sure that the secrets manager settings is hidden behind the secrets_manager feature flag, and not just the feature license. Owners should be able to make changes, while Maintainers have view-only permissions.

Backporting this bug fix to 18.3 is based on the discussion in !203977 (comment 2732867438). The original MR (!203977 (merged)) has been merged to the default branch in 18.4.

The secrets manager feature is a WIP so this does not impact functionality. However, API error message that we provide to the users, detailed in #555427 (comment 2729188083), is confusing and can cause them to be unsure whether the changes they've made to the settings have been successful or not.

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).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• 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:test-on-omnibus-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).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.