fix: Change Default Logo from .ico to .png in NuGet

[garethpaul]

Fixes

On Visual Studio the current icon for twilio-csharp appears to be scrambled.

This change modifies the .ico file with .png.

Note: I wasn't entirely sure how to test this given that this is hosted with NuGet, but perhaps there is additional functionality that can be utilized for testing.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the master branch
• [] I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.