feat(lsp): deprecate severity_limit

Problem:
  `vim.lsp.diagnostic.on_diagnostic` accepts an undocumented severity_limit
  option which is widely used.

Solution:
  Deprecate it in favour of `{min = severity}` used in `vim.diagnostic`.
  Since this is undocumented, the schedule for removal is accelerated to
  0.11.

runtime/doc/lsp.txt

@@ -1384,6 +1384,7 @@ on_diagnostic({_}, {result}, {ctx}, {config})
 <
 
     Parameters: ~
+      • {result}  (`lsp.DocumentDiagnosticReport`)
       • {ctx}     (`lsp.HandlerContext`)
       • {config}  (`table`) Configuration table (see
                   |vim.diagnostic.config()|).