Remove use of deprecated vim.tbl_flatten

[reentim]

I'm attempting to move across to Neovim (using v0.11), and in trying to get a clean bill of of checkhealth :

WARNING vim.tbl_flatten is deprecated. Feature will be removed in Nvim 0.13
- ADVICE:                                                                  
  - use vim.iter(…):flatten():totable() instead.                           

Not sure if there's something I'm overlooking, though?

[wincent]

Thanks @reentim!

Not sure if there's something I'm overlooking, though?

Looks good to me. The default flatten() method will unnest by 1 level, which is what we want here (we have a list of lists of errors, and we want to turn it into a list of errors).

Tested that it works by passing some invalid configs and seeing the error messages come through correctly. Also, by adding some print(vim.inspect(...)) debug staements to see that the flattening was happening as expected. Will merge.