MCP Server should be behind experiment & beta features setting

Description

MCP server is marked as an experimental feature. It is currently only gated by a feature flag.

• https://gitlab.com/gitlab-org/gitlab/-/blob/7cf1c5c7f2a2a18228652c1539409e50de3773cd/lib/api/mcp/server.rb#L44
• https://gitlab.com/gitlab-org/gitlab/-/blob/7cf1c5c7f2a2a18228652c1539409e50de3773cd/lib/api/mcp/base.rb#L46

Proposal

While the MCP server is in Experiment or Beta product phase, we'll need to ensure that it is only accessible after customers have opted into our Experiment/Beta features.

• Customers need to opt into Feature preview in their Duo Configuration.
• Only after the Feature preview has been enabled and the feature flags have been enabled, they can the user use the GitLab MCP server.
• On GitLab SaaS, follow the experiment & beta features setting at the root group level.
• On GitLab Self-managed, follow the experiment & beta features setting at the instance level.