Go back to returning nil for cancelled requests (#3024)

### Motivation

This is a temporary revert to fix #3019 while we understand the correct response flow for retried cancelled requests.

This PR reverts the behaviour introduced in #2939, which didn't cause problems for NeoVim.

### Implementation

I didn't perform a full revert of every single change, since we do want to ensure that we're following the spec. I just stopped returning the error and went back to returning a `nil` response until we figure out what's going on.

### Automated Tests

Changed the test so that it asserts the current behaviour. We'll switch it back later.

lib/ruby_lsp/base_server.rb

@@ -158,11 +158,7 @@ def new_worker
           # Check if the request was cancelled before trying to process it
           @global_state.synchronize do
             if id && @cancelled_requests.include?(id)
-              send_message(Error.new(
-                id: id,
-                code: Constant::ErrorCodes::REQUEST_CANCELLED,
-                message: "Request #{id} was cancelled",
-              ))
+              send_message(Result.new(id: id, response: nil))
               @cancelled_requests.delete(id)
               next
             end

test/server_test.rb

@@ -751,7 +751,7 @@ def test_requests_to_a_non_existing_position_return_error
     assert_match("Request textDocument/completion failed to find the target position.", error.message)
   end
 
-  def test_cancelling_requests_returns_expected_error_code
+  def test_cancelling_requests_returns_nil
     uri = URI("file:///foo.rb")
 
     @server.process_message({
@@ -791,9 +791,8 @@ def test_cancelling_requests_returns_expected_error_code
     mutex.unlock
     thread.join
 
-    error = find_message(RubyLsp::Error)
-    assert_equal(RubyLsp::Constant::ErrorCodes::REQUEST_CANCELLED, error.code)
-    assert_equal("Request 1 was cancelled", error.message)
+    result = find_message(RubyLsp::Result)
+    assert_nil(result.response)
   end
 
   def test_unsaved_changes_are_indexed_when_computing_automatic_features