GitlabPHP client update 12.0.0 -> 12.1.0

[TafkaMax]

Finally a new release, which unblocks atleast 1 previous PR. https://github.com/GitLabPHP/Client/releases/tag/12.1.0

Could we get this update merged in depndencies?

[Adambean]

Existing PR for this: #52
Probably a duplicate of this issue: #50