refactor(lsp): replace util.buf_versions with changedtick

[mfussenegger]

lsp.util.buf_versions was already derived from changedtick (on_lines
from buf_attach synced the version)

As far as I can tell there is no need to keep track of the state in a
separate table.

[icholy]

FYI: https://github.com/neovim/nvim-lspconfig/blob/2c1877081b237a643e52ebdebaf36c84a2695639/lua/lspconfig/server_configurations/eslint.lua#L29

Also: https://sourcegraph.com/search?q=context:global+vim.lsp.util.buf_versions&patternType=keyword&sm=0

Could do something like

---@nodoc
---@deprecated
M.buf_versions = setmetatable({}, {
  __index = function(_, bufnr)
    vim.notify_once("vim.lsp.util.buf_versions is deprecated", vim.log.levels.WARN)
    return vim.b[bufnr].changedtick
  end
})

[icholy]

Might make sense to hide the vim.b[bufnr].changedtick inside a util.buf_version(bufnr) function:

function M.buf_version(bufnr)
  return vim.b[bufnr].changedtick
end

This way it can be changed in the future without breaking anything.

[mfussenegger]

Might make sense to hide the vim.b[bufnr].changedtick inside a util.buf_version(bufnr) function:

function M.buf_version(bufnr)
  return vim.b[bufnr].changedtick
end

This way it can be changed in the future without breaking anything.

I don't think that's necessary and it would defeat the purpose of this PR (getting rid of unnecessary extra API).
util.buf_versions wasn't intended as public API (hence the @nodoc) and I was even inclined to skip the deprecation step because of that.