[Task] Deprecated raw-password auth path still exported on public service interface

## Description
All three `@Deprecated` ("do not use … for authentication with raw passwords"). The impl still calls the mapper directly. `PasswordHashService` is the replacement but the raw-password entry point remains on a public service interface with no removal version.

## Location
```
shenyu-admin/.../mapper/DashboardUserMapper.java:69
.../service/DashboardUserService.java:91
.../service/impl/DashboardUserServiceImpl.java:266
```

## Impact
Misleading API surface; security-sensitive deprecation without removal target.

## Suggested fix
Set a removal version; gate the impl to delegate to `PasswordHashService`; or remove the public method.

## Related existing issue(s)
Adjacent to #5255/#4602 (closed password-security tasks) but this is the lingering deprecated public surface.

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Start with DashboardUserMapper.java:69, DashboardUserService.java:91, and DashboardUserServiceImpl.java:266. Compare the deprecated raw-password path with PasswordHashService and review the related issues before choosing whether to set a removal version, delegate through the replacement, or remove the public method. Done means the selected policy is applied consistently to the public interface and implementation.

Written by the indexing model from the issue text.