[3.x] Add validator related deprecation notices

[tambait]

Subject

I am targeting this branch, because BC.

Changelog

### Deprecated

- `AbstractAdmin::$validator` property
- `AdminInterface::getValidator` method specification
- `AdminInterface::setValidator` method specification
- `AbstractAdmin::getValidator` method implementation
- `AbstractAdmin::setValidator` method implementation
- `AddDependencyCallsCompilerPass` - validator related keys
- `DependencyInjection/Configuration` - validator related config option

[VincentLanglet]

This method is automatically called by the CompilerPass, so I think we can't trigger an error inside (or everybody will have a deprecation everytime). @sonata-project/contributors ?

[VincentLanglet]

Didn't see the tests ; do you know how to fix them ?

• For the setValidator, I think you should remove the trigger_error
• For the getValidator, Use @group legacy, NEXT_MAJOR: remove this test and add $this->expectDeprecation

[VincentLanglet]

One last thing, you have to update the UPGRADE note file, and the PR will be perfect

[tambait]

Didn't see the tests ; do you know how to fix them ?

Thanks. I wouldn't know how to fix tests without your help.

[franmomu]

I think there is also this one:

SonataAdminBundle/tests/Admin/AdminTest.php

Lines 2841 to 2843 in c30d61e

	$validator = $this->createMock(ValidatorInterface::class);
	$validator->method('getMetadataFor')->willReturn($this->createStub(MemberMetadata::class));
	$admin->setValidator($validator);

[SonataCI]

Could you please rebase your PR and fix merge conflicts?

[SonataCI]

Could you please rebase your PR and fix merge conflicts?