Replace Auto Devops Banner with unobtrusive banner similar to service desk

See for more information https://gitlab.com/gitlab-org/gitlab-ce/issues/38931

Design

Current banner will be replaced with the following.

Note: this issue is only about changing the appearance of the banner, it will apply in any place where the banner is shown. Discussion on where the banner will appear is in https://gitlab.com/gitlab-org/gitlab-ce/issues/38827 and https://gitlab.com/gitlab-org/gitlab-ce/issues/39051.