Remove .webp -> .png conversion in opengraph image generation

[busbyk]

satori which is used under the hood for vercel/og doesn't support rendering .webp images. So we have to convert .webp images to .png in the image generation logic. This is obviously not ideal but this functionality should land soon: vercel/satori#622

See TODOs in src/utilities/generateOGImage.tsx and this PR where they were introduced #395