audit: disallow unstable specs for versioned formulae

Library/Homebrew/dev-cmd/audit.rb

@@ -571,8 +571,19 @@ module Homebrew
         end
       end
 
-      if @new_formula && formula.head
-        new_formula_problem "Formulae should not have a HEAD spec"
+      if formula.head || formula.devel
+        unstable_spec_message = "Formulae should not have an unstable spec"
+        if @new_formula
+          new_formula_problem unstable_spec_message
+        elsif formula.versioned_formula?
+          versioned_unstable_spec = %w[
+            bash-completion@2
+            imagemagick@6
+            openssl@1.1
+            python@2
+          ]
+          problem unstable_spec_message unless versioned_unstable_spec.include?(formula.name)
+        end
       end
 
       throttled = %w[