Use .svg instead of .png

[steren]

The button looks a bit pixelated as PNG. Since it is a vector graphic in the first place, we could create and use https://storage.googleapis.com/cloudrun/button.svg

I can take care of that, I probably have the asset on my machine.

[ahmetb]

@steren can you upload the svg sometime? I'd love to use it.

[steren]

I uploaded the SVG, it should be of the appropriate size: https://storage.googleapis.com/cloudrun/button.svg

[ahmetb]

@steren we should stop making changes to this repo. New changes should go to
https://github.com/GoogleCloudPlatform/cloud-run-button.