I noticed I had to replace the deprecated rubocop-lsp with ruby-lsp in both my .vscode/extensions.json and Gemfile. After a bundle install and restarting VSCode, it was still showing me this warning:

I wasn't sure what it was about. I thought I had misconfigured my project's ruby version but wasn't the case. After asking around, it was solved by a bundle update ruby-lsp which pulled the latest version 0.3.3 and removed the warning.
Perhaps the warning/error message can be more descriptive for cases like this?