ruby-build: definition not found: x.x.x

I recently came across an error when trying to install a new version of ruby using asdf.

$ asdf install ruby 2.7.5
ruby-build: definition not found: 2.7.5

So I thought, oh, I need to update ruby-build to get the latest list of available versions, so I try to upgrade ruby-build and try again.

$ brew upgrade ruby-build
...
$ asdf install ruby…