Drop support for outdated versions of Neovim

[dcampos]

Problem Statement

Currently, Snippy supports Neovim >= 0.5.0. Some features added in later versions, like Lua autocmds, may improve performance and make the code more readable.

Ideas or possible solutions

The idea is to make 0.7.0 the minimum compatible version, while possibly keeping a separate nvim-0.5.0 branch for those who aren't able to upgrade Neovim to a more recent version.

Alternatives you have considered

Neovim development is progressing at a fast speed, so keeping compatibility with outdated versions doesn't seem necessary.