Fix all phpstan-drupal reports

Up to level 2 included.

Up to level 5 included.

Up to level 6 included.

Merged to current dev HEAD.

Actually, only the tests were fixed. More work needed.

Everything passing locally, but now phpcs complains that one should not redocument params.

Tip: using @phpstan-param and @phpstan-return fixes it locally.

Requiring coder 8.x-3.20 to avoid the silly return type warnings with 8.3.16.

Head now passes at level 6. On to the next levels.

Fixed up to level 8.

fgm → closed merge request !6

Should be complete.

Merged to current dev HEAD.

Automatically closed - issue fixed for 2 weeks with no activity.