Deprecate "staging_upload_user" and "staging_upload_password"

Issue

Deprecate the staging.auth.user and staging.auth.password credentials. They are used to protect some internal endpoints, but those have been secured with mTLS in the meantime.

Background: We need to ensure that all passwords are rotated regularly. We've enabled a password rotation and detected no downtimes for cf push or any other operation. If the credentials are not really used anymore, it would of course be better to remove them entirely.

https://github.com/cloudfoundry/cloud_controller_ng/blob/dbeb9dbdeed152cbaf88d4bac29299fa9c26ce9d/config/cloud_controller.yml#L151

Context

The staging_upload_user/staging_upload_password are used to protect the following endpoints with basic authentication:

1. GET /staging/packages/:guid
   https://github.com/cloudfoundry/cloud_controller_ng/tree/main/docs/internal#get-stagingpackagesguid

2. GET /staging/v3/droplets/:guid/download
   https://github.com/cloudfoundry/cloud_controller_ng/tree/main/docs/internal#get-stagingv3dropletsguiddownload

3. GET /staging/v3/buildpack_cache/:stack/:app_guid/download
   https://github.com/cloudfoundry/cloud_controller_ng/tree/main/docs/internal#get-stagingv3buildpack_cachestackapp_guiddownload

The three endpoints above are only used for the "NFS server mounted on the CC" scenario. Is this still a supported setup?

4. GET /staging/jobs/:guid
   https://github.com/cloudfoundry/cloud_controller_ng/blob/ec8558268aa59131580a050f8870b8a5f5f69c57/app/presenters/api/staging_job_presenter.rb

Looks like dead code as the http scheme is not used anymore.

5. GET /v2/buildpacks/:guid/download
   https://github.com/cloudfoundry/cloud_controller_ng/tree/main/docs/internal#get-v2buildpacksguiddownload
   https://github.com/cloudfoundry/cloud_controller_ng/blob/ec8558268aa59131580a050f8870b8a5f5f69c57/app/controllers/runtime/buildpack_bits_controller.rb#L10

Not in use anymore as Diego downloads directly from blobstore?

6. blobstore_url_generator
   https://github.com/cloudfoundry/cloud_controller_ng/blob/ec8558268aa59131580a050f8870b8a5f5f69c57/lib/cloud_controller/dependency_locator.rb#L190

This function is used in different places:

https://github.com/cloudfoundry/cloud_controller_ng/blob/ec8558268aa59131580a050f8870b8a5f5f69c57/app/controllers/runtime/stagings_controller.rb
-> no use of blobstore_url_generator

https://github.com/cloudfoundry/cloud_controller_ng/blob/ec8558268aa59131580a050f8870b8a5f5f69c57/app/controllers/internal/download_droplets_controller.rb#L17
For internal endpoint GET /internal/v4/droplets/:guid/:droplet_checksum/download:
https://github.com/cloudfoundry/cloud_controller_ng/tree/main/docs/internal#get-internalv4dropletsguiddroplet_checksumdownload
-> in use, but without basich auth?

Start with config/cloud_controller.yml at the staging credentials and trace their uses through the listed staging, buildpack, droplet, and blobstore endpoints, including lib/cloud_controller/dependency_locator.rb and the referenced controllers and presenter. Confirm whether the NFS-mounted CC setup and each endpoint remain supported, then define the deprecation or removal scope and update the affected documentation and behavior accordingly.