[4.x] Update to enhance phpstan level

[BinaryKitten]

Description

On stream with nuno - there was a request to update the phpstan level. I posted "Consider it done.." in a sort of "Challenge accepted" manner.

This mostly adds docblocks to denote types - but a few minor code changes have been made to remove existing level 6 issues.

Currently this is a Work in progress, thus the draft status - more commits as i find time. Once i've squashed all the level 6 issues - then I'll move forward to Level 7+ each as a new PR.