Reasons for creating this PR

I noticed that Skosmos 3.0 was still using the old favicon (boxy S-shaped icon) that does not match the Skosmos 3 style and colors. This PR updates the icon to the new design (by Hahmo).

Here are the old and new favicons side by side in Firefox:

As before, the default favicon may be overridden by adding a custom-favicon.ico file in the root directory that will then be used instead of the default.

Link to relevant issue(s), if any

n/a

Description of the changes in this PR

Set new favicon

Known problems or uncertainties in this PR

None

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)