Don't set vim.o.termguicolors = true

[Jendker]

Setting vim.o.termguicolors = true in latest nightly causes the background color to change temporarily during loading. By default background is not going to be set temporarily if we usevim.o.termguicolors = false. This has been discussed in this issue https://github.com/neovim/neovim/issues/26372 and wasn't really resolved.

See videos below. Without termguicolors:

https://github.com/navarasu/onedark.nvim/assets/14967831/b4ce1ff2-4ff0-4eb2-a15f-ac7c4280831b

With termguicolors:

https://github.com/navarasu/onedark.nvim/assets/14967831/b0807aa6-e1e0-4599-9671-6481f8556c3b

By not setting termguicolors = true we comply with the current requirements for neovim for consistent background. If the user needs the termguicolors = true they can always set it in their config. I am not sure why this setting was needed in this plugin in the first place, though. Please let me know if I am missing something.