Remove image banner support

[philwebb]

Image banners rely on awt classes to convert the gif/png/jpeg file to text. They also convert the image on each run of the application. It would probably be better to remove support and instead offer a tool that can be run once to convert an image to a banner files. It would also help solve the "headless" issue identified in #28803

[rudy2steiner]

Let me try this!

[wilkinsona]

I have removed support for image-based banners. A replacement tool has not been added as it's not clear to me that there is sufficient demand for one in Spring Boot itself. We could consider adding such a tool to the CLI if there's sufficient demand in the future.

[joshlong]

I understand the reasoning. It makes good sense. But I am sad.