Option to Disable Username/Password Authentication

### Current Behavior

**Problem:**
We aim to improve the security posture of our Dependency-Track instance by eliminating the option for username/password login. We want to enforce the use of stronger authentication methods such as OpenID Connect.

**Proposed Solution:**
Implement a configuration setting that allows administrators to completely disable username/password-based authentication. When this setting is active, users should only be able to log in via other configured authentication mechanisms (e.g., OpenID Connect).

**Context:**
Disabling traditional username/password login aligns with security best practices and mitigates risks associated with credential compromise.

### Proposed Behavior

Implement a configuration setting that allows administrators to completely disable username/password-based authentication. When this setting is active, users should only be able to log in via other configured authentication mechanisms (e.g., OpenID Connect).

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested