Drop support for neovim 0.9

[Shaobin-Jiang]

While plenty of people still use neovim < 0.10, I myself exclusively use neovim 0.10.x on all of my devices. Maintaining the config for an older version of neovim that is not easily available to me is becoming more and more inconvenient.

The current plan is, upon the release of neovim 0.11 and after I have upgraded to the latest version, support for neovim 0.9 will be dropped, as will the support for all but the latest stable release.

Should be easy, as currently all the code for neovim 0.9 compatibility lies in v0_9.lua and all that needs to be done is removing the file and the code that requires it in init.lua.