Fix: lsp.start_client() deprecated

[okGus]

In Neovim 0.13, feature vim.lsp.start_client() is deprecated. Using vim.lsp.start() instead.

[ViViDboarder]

I'm pretty sure that this change isn't backwards compatible. A version check should probably be made or checking to see if the value is in the table.

[xunoaib]

This unfortunately doesn't work for me on Neovim 0.12. I get [LLM] Error starting llm-ls. Replacing vim.lsp.start_client with vim.lsp.start causes the call to return a nil client ID, and the buffer is never attached. However, :LspInfo still shows llm-ls as running in the background, just not attached to the current buffer.

I did some preliminary debugging, but had trouble making sense of the docs to figure out what we should be doing differently.