Remove gradients from custom tailwind config

[pixelastic]

This has been integrated into the core for a few minor versions already: tailwindlabs/tailwindcss#2176

This is not the same syntax as the one I used, so simply removing it will be a breaking change. I don't think I have a lot of websites using this syntax so I could go with a minor update (so that renovate will catch it).

I think the only one using it are:

• Sov, for the progressbar but it will get revamped anyway
• Monsters and Society, I think they use this for adding smoother semi-transparent layer on the sidebar and header shadow