Update installer branding

[td202]

Purpose

Update the background images used in the Windows and Mac installers to reflect the updated Concordium branding.

Changes

• Replace Windows installer welcome background and banner background.
• Replace Mac installer background.

Checklist

• [ ] My code follows the style of this project.
• [ ] The code compiles without warnings.
• [ ] I have performed a self-review of the changes.
• [ ] I have documented my code, in particular the intent of the hard-to-understand areas.
• [ ] (If necessary) I have updated the CHANGELOG.