break: bump the minimum required neovim version to 0.6 or higher (#1549)

So we can use `vim.json` and `vim.diagnostics`.

If you aren't able to update to 0.6.* yet you should pin the previous commit.

plugin/telescope.vim

@@ -1,5 +1,5 @@
-if !has('nvim-0.5.1')
-  echoerr "Telescope.nvim requires at least nvim-0.5.1. See `:h telescope.changelog-1406`"
+if !has('nvim-0.6.0')
+  echoerr "Telescope.nvim requires at least nvim-0.6.0. See `:h telescope.changelog-1549`"
   finish
 end