fix: use nil for the default diagnostics severity (#2662)

* fix: use nil for the default diagnostics severity

https://github.com/neovim/neovim/pull/24736 has introduced a new syntax
for severity format in `vim.diagnostic.get()`.

Fix #2661

* fix: fallback for 0.9.1

* fix: detect changes only when minor version > 9

(cherry picked from commit 6b79d7a6a4)

lua/telescope/builtin/__diagnostics.lua

@@ -44,6 +44,9 @@ local diagnostics_to_tbl = function(opts)
       diagnosis_opts.severity["max"] = opts.severity_bound
     end
   end
+  if vim.version().minor > 9 and vim.tbl_isempty(diagnosis_opts.severity) then
+    diagnosis_opts.severity = nil
+  end
 
   opts.root_dir = opts.root_dir == true and vim.loop.cwd() or opts.root_dir