(maint) Revise check for unsafe_load_file

It's possible to install Psych 5 with Ruby 2.7 causing YAML.load_file to call
the safe version. So revise the check based on whether the unsafe_load_file
method exists, and remove it when Ruby < 3.1 is dropped.

Author: joshcooper

spec/unit/reports/store_spec.rb

@@ -15,10 +15,12 @@
     end
 
     let(:report) do
-      if RUBY_VERSION < "3.0"
-        report = YAML.load_file(File.join(PuppetSpec::FIXTURE_DIR, 'yaml/report2.6.x.yaml'))
-      else
+      # It's possible to install Psych 5 with Ruby 2.7, so check whether underlying YAML supports
+      # unsafe_load_file. This check can be removed in PUP-11718
+      if YAML.respond_to?(:unsafe_load_file)
         report = YAML.unsafe_load_file(File.join(PuppetSpec::FIXTURE_DIR, 'yaml/report2.6.x.yaml'))
+      else
+        report = YAML.load_file(File.join(PuppetSpec::FIXTURE_DIR, 'yaml/report2.6.x.yaml'))
       end
       report.extend(described_class)
       report