Rename gitlab:update parameter to gitlab:keep-history and make it mandatory

Current situation

By default, when the quality gate results are sent to GitLab, the results history is kept: to update it for a given quality gate, the user must set the --plugin gitlab:update parameter.

Desired outcome

The gitlab:update parameter becomes gitlab:keep-history to harmonize the naming between tools and tracker publisher plugin. It also becomes mandatory, forcing the user to choose if he wants to keep quality gate results history or no.