[Rails] Add example OpenBao URL configuration

See first bullet in this issue comment: https://gitlab.com/gitlab-org/govern/secrets-management/deployment-investigation/-/issues/8#note_2482009632

In the Rails config example: https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/gitlab.yml.example?ref_type=heads#L1454

An openbao section used to exist when proxy_address was included. This was removed when proxy was removed but we should include a commented-out example configuration with the defaults, so people know these exist and can be modified:

# openbao:
#   url: https://localhost:8200

This improves DevEx for internal teams. Related: #540874 (closed)

&c.