Merge ci_tanukey_ui feature flag with secrets_manager

Why are we doing this work

See #474432 (comment 2557919450).

We have separate frontend and backend feature flags for the Secrets Manager. This was from when frontend started working on it first and both frontend and backend worked in parallel.

There is no need to maintain both feature flags, so we can just merge them together in secrets_manager. This will also make it easier to manage which projects are included in the Closed Experiment and Beta.

Relevant links

[Feature flag] Enable secrets_manager (#474432)

[Feature flag] Enable `ci_tanukey_ui` feature flag (#427026 - closed) (to be removed)

Non-functional requirements

• Documentation
• Feature flag
• Testing

Implementation plan

• Update the code to replace usage of ci_tanukey_ui with secrets_manager
• Remove the feature flag and clean them up from the environments
• Remove references to ci_tanukey_ui in the Openbao Setup Instructions

Verification steps

• There are no references to ci_tanukey_ui in the code
• Pipelines should pass (all tests work as expected) and existing features are not broken