Add ruby 2.6.8, remove ruby 2.6.6
Add ruby 2.6.8, remove ruby 2.6.6

for stack(s) cflinuxfs3 [#178800606]

cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack