fix(admin): update branding assets

[ptrckbp]

Description

Updates branding assets to match new ones in the admin panel. Corresponding PR for the studio assets is inbound. Also removes unused image. Note that the auth page logo has already been converted.

New assets :
favicon :

admin panel icon :

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [x ] Bug fix (non-breaking change which fixes an issue)

How has this been tested?

Manually tested.

favicon tested under dark blue and white background conditions. It looks nicer than the black on white version.

Checklist:

• [ x] My code follows the style guidelines of this project
• [x ] I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules
• I've included some media (picture/gif/video) if applicable to show the old and new behavior

[laurentlp]

Tested and LGTM!

[laurentlp]

Nevermind my last comment, it seems like the favicons are missing when clicking Open Chat on any bot from the Admin page

[ptrckbp]

I updated the link to the correct location (favicon now shows in "Open Chat" path) and searched for more versions of it, but couldn't find some, so I think it's solved.