Use validateData() to validate POST data only

[obozdag]

Instead of withRequest(), use validateData() to validate POST data only. withRequest() uses $request->getVar() which returns $_GET, $_POST and $_COOKIE data in that order. Newer values override older values. Post values will be overriden by the cookies if they have the same name.

Description
Explain what you have changed, and why.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

Duplicate of #8157

[obozdag]

user_guide_src/source/libraries/validation.rst and user_guide_src/source/incoming/controllers.rst files have similar warnings. That is why I have changed those two files.

[kenjis]

I thought the same submission was sent twice because it was the same subject.