Context, User: deprecate `validateCredentials` in `UserElement` classes.

[paulgibbs]

Description

Remove validateCredentials methods in UserElement classes. This attempted to compare passwords on user objects.

Methods will be removed in 4.0.0.

Related issue

See #155

Motivation and context

I've decided I do not like behaviour that cannot be specific to all drivers, unless there is a strong reason, and I do not think this has it. I have been skeptical of the benefit of trying to compare user passwords in this method, so I am going to rip it out.

How has this been tested?

Not locally. :D

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.