[Meta] Fix 'SlevomatCodingStandard.TypeHints.DeclareStrictTypes' coding standard

mstrlen reminded me that this should be postponed on 🌱 [Meta] Implement strict typing in existing code Active

The Coder issue to add the sniff is ✨ Add sniff for declare(strict_types=1); Needs review . When that is done and a new coder release made that is used in core 11.x then work can be started on it.

The sniff only reports errors in the formatting of the declare statement. We have turn off the option to enforce strict typing (for now) so it will simply report on the files where the declare statement is not written correctly.