Fix an error when using built-in LSP

This PR fixes the following error when using built-in LSP with Ruby 3.4dev (default by Prism).

LSP log (eglot-events-buffer):

```console
[jsonrpc] e[04:03:30.163] --> workspace/didChangeConfiguration {"jsonrpc":"2.0","method":"workspace/didChangeConfiguration","params":{"settings":{}}}
[stderr]  [server] RuboCop 1.66.1 language server +YJIT initialized, PID 5753
(snip)

[stderr]  [server] Error NameError uninitialized constant RuboCop::LSP::Runtime::StringIO
[stderr]  [server] ["/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/lsp/runtime.rb:85:in 'RuboCop::LSP::Runtime#redirect_stdout'",
```

Ruby version:

```console
$ ruby -v
ruby 3.4.0dev (2024-09-19T18:51:21Z master 2230ac4a28) +PRISM [x86_64-darwin23]
```

lib/rubocop/lsp/runtime.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'stringio'
+
 #
 # This code is based on https://github.com/standardrb/standard.
 #