refactor!: replace util.path.join with vim.fs.joinpath

[dundargoc]

We should rely on builtin functionality if possible to minimize code duplication.

[dundargoc]

Unsure what general strategy is for removing lspconfig functions. This seems used fairly frequently: https://github.com/search?q=util.path.join+language%3ALua&type=code. Worth deprecating first, or do we just go for it?

[justinmk]

what general strategy is for removing lspconfig functions

we should keep the util stuff around to avoid breaking downstream. meanwhile, in this repo:

• CI should fail if any config uses a deprecated util function
• we should remove all docs that suggest a deprecated util function

once we have isolated most of util in that way, then we can think about when/how to fully remove them.

#2079 (comment)

[justinmk]

To be clear, my above comment is in favor of this PR. When you asked about "removing lspconfig functions" I interpreted that to mean removing the util.xx entirely

OTOH #3342 is making me think that 0.9 support is still worthwhile, if it doesn't add too much burden.