fix(gitlab): remove double call for MR assignees (#14212)

* Fix primitive assignee calls

* Use proper Gitlab pronoun

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

* Use proper Gitlab offering noun

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

* Apply requirements

* Fix addAssignees test

* Remove debug log

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

* Implement getQueryString in addAssignees

* Add note on multiple assignees to platform/gitlab.md

* Fix typo

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

* Update lib/platform/gitlab/index.md

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>