Drop rdoc from test dependencies
rdoc isn't really needed in testing, only during development. This reduces the number of gems that need to be installed to test and is closer to production where rdoc isn't installed either.
Refs #34563 - Fix rdoc task to refer to README.md
In c08eb9a80d33655279ad4cc6039f0299ccb14e3b this file was renamed from
README to README.md, but this reference wasn't updated.