[cleanup] deprecate SHOULD_KEEP_PRE_SLASH, remove dont traverse const on StringClassNameToClassConstantRector

[TomasVotruba]

No description provided.

[samsonasik]

If configurable removed, the \\ string prefix should be added if string has \\ prefix, eg:

"\Foo\Bar"

to be:

"\" . \Foo\Bar::class

so that value become equal, the use of \\ prefix is needed.

That's we keep the value without breaking change.

[TomasVotruba]

I know. The purpose of this rule is to remove these edge-cases too.