Update obsolete-banner in accordance with design guidelines

Currently, the obsolete banner looks as:

However, verifying the design guidelines with the team, the following is required:

The changes in this MR ensures that.

Note: There is still an issue with websites already containing a cern-toolbar element which would then be duplicated, though for many this is not the case. One example of a website already containing the cern-toolbar and thus exhibiting the above problem is https://cern60-el9-preview.webtest.cern.ch/cern60/. On the other hand, a website like https://cern-discoveries.web.cern.ch/ does not already have a cern-toolbar and look perfectly fine. This might be something we can discuss.