fix(lsp): log unknown diagnostic tags instead of showing a warning (#25705)

To be more in line with the specification:

> To support the evolution of enumerations the using side of an enumeration shouldn’t fail on an enumeration value it doesn’t know. It should simply ignore it as a value it can use and try to do its best to preserve the value on round trips

runtime/lua/vim/lsp/diagnostic.lua

@@ -2,6 +2,7 @@
 
 local util = require('vim.lsp.util')
 local protocol = require('vim.lsp.protocol')
+local log = require('vim.lsp.log')
 local ms = protocol.Methods
 
 local api = vim.api
@@ -88,10 +89,7 @@ local function tags_lsp_to_vim(diagnostic, client_id)
       tags = tags or {}
       tags.deprecated = true
     else
-      vim.notify_once(
-        string.format('Unknown DiagnosticTag %d from LSP client %d', tag, client_id),
-        vim.log.levels.WARN
-      )
+      log.info(string.format('Unknown DiagnosticTag %d from LSP client %d', tag, client_id))
     end
   end
   return tags