Add ruby 2.6.9, remove ruby 2.6.7
Add ruby 2.6.9, remove ruby 2.6.7

for stack(s) cflinuxfs3 [#180426021]

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