Double-check date/time mapping support for immutable types

[mbabker]

Scanning for "datetime" in the extensions, there are a couple of places where there are lists of date-related mapping types checked for different features:

• Gedmo\Translatable\Query\TreeWalker\TranslationWalker checks for the mutable types but doesn't check the immutable counterparts
• Gedmo\Tree\Mapping\Validator::isValidFieldForLockTime() only supports mutable types for the lock time

Unless there's a strong reason to only support the mutable variant, the immutable types should probably be supported as well.