That's a weird one, ServletRequest.setAttribute(String, Object) is a valid (and very old) method. You must have a plugin that's contributing something funky - does the output of dependency-report show an unusual version of the servlet API jar?
Ok, so removing the scaffolding plugin maybe isn't the best idea... so after some more research and experimenting... I discovered that the version in the scaffold plugin was the wrong version. I'm upgrading a 2.3.7 grails app to 2.5.6.
Perhaps your versions are incorrect, too. I discovered the correct version by creating a new grails app and inspecting BuildConfig.groovy.