fix(lsp): add RequestFailed error code constant #34645

Also remove `serverErrorStart/End` as [the spec](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#errorCodes)
says that they're deprecated and don't even represent a real error code.

runtime/lua/vim/lsp/protocol.lua

@@ -162,14 +162,13 @@ local constants = {
     MethodNotFound = -32601,
     InvalidParams = -32602,
     InternalError = -32603,
-    serverErrorStart = -32099,
-    serverErrorEnd = -32000,
     ServerNotInitialized = -32002,
     UnknownErrorCode = -32001,
     -- Defined by the protocol.
     RequestCancelled = -32800,
     ContentModified = -32801,
     ServerCancelled = -32802,
+    RequestFailed = -32803,
   },
 
   -- Describes the content type that a client supports in various