.fixtures.yml: pin yumrepo_core to v2.1.0

the puppetlabs/postgresql CI still tests on Puppet 7. yumrepo_core
v3.0.0 doesn't support it anymore.

Author: bastelfreak

.fixtures.yml

@@ -10,5 +10,7 @@ fixtures:
     provision: "https://github.com/puppetlabs/provision.git"
     puppet_agent: 'https://github.com/puppetlabs/puppetlabs-puppet_agent.git'
     stdlib: "https://github.com/puppetlabs/puppetlabs-stdlib.git"
-    yumrepo_core: "https://github.com/puppetlabs/puppetlabs-yumrepo_core.git"
+    yumrepo_core:
+      repo: "https://github.com/puppetlabs/puppetlabs-yumrepo_core.git"
+      ref: "v2.1.0"
     systemd: "https://github.com/voxpupuli/puppet-systemd.git"