Update GitLab logo to new branding in Grafana dashboard quickstart

[dnsmichi]

The Grafana quickstart dashboards are great for getting things going in the docker compose demo. This PR updates the GitLab logo to the new branding, using the official images from the brand presskit URLs that are publicly available.

During tests, I have also changed the image width to 100px instead of 80px, better fitting the widgets.

cc @mvisonneau

[coveralls]

coverage: 75.617%. remained the same when pulling 4c9be772b8791dacf7e921b07492227857f9facd on dnsmichi:update-gitlab-logo-grafana into 5e1c3b73d9c899b2201fba30aadf09055fb79a30 on mvisonneau:main.

[mvisonneau]

thanks @dnsmichi! 🙌