Deprecate taint/trust and related methods, and make the methods no-ops

This removes the related tests, and puts the related specs behind
version guards.  This affects all code in lib, including some
libraries that may want to support older versions of Ruby.

spec/ruby/core/array/uniq_spec.rb

@@ -39,44 +39,76 @@ describe "Array#uniq" do
     [x, y].uniq.should == [x, y]
   end
 
-  it "compares elements with matching hash codes with #eql?" do
-    a = Array.new(2) do
-      obj = mock('0')
-      obj.should_receive(:hash).at_least(1).and_return(0)
+  ruby_version_is '2.7' do
+    it "compares elements with matching hash codes with #eql?" do
+      a = Array.new(2) do
+        obj = mock('0')
+        obj.should_receive(:hash).at_least(1).and_return(0)
 
-      def obj.eql?(o)
-        # It's undefined whether the impl does a[0].eql?(a[1]) or
-        # a[1].eql?(a[0]) so we taint both.
-        taint
-        o.taint
-        false
+        def obj.eql?(o)
+          false
+        end
+
+        obj
       end
 
-      obj
-    end
+      a.uniq.should == a
 
-    a.uniq.should == a
-    a[0].tainted?.should == true
-    a[1].tainted?.should == true
+      a = Array.new(2) do
+        obj = mock('0')
+        obj.should_receive(:hash).at_least(1).and_return(0)
 
-    a = Array.new(2) do
-      obj = mock('0')
-      obj.should_receive(:hash).at_least(1).and_return(0)
+        def obj.eql?(o)
+          true
+        end
 
-      def obj.eql?(o)
-        # It's undefined whether the impl does a[0].eql?(a[1]) or
-        # a[1].eql?(a[0]) so we taint both.
-        taint
-        o.taint
-        true
+        obj
       end
 
-      obj
+      a.uniq.size.should == 1
     end
+  end
 
-    a.uniq.size.should == 1
-    a[0].tainted?.should == true
-    a[1].tainted?.should == true
+  ruby_version_is ''...'2.7' do
+    it "compares elements with matching hash codes with #eql?" do
+      a = Array.new(2) do
+        obj = mock('0')
+        obj.should_receive(:hash).at_least(1).and_return(0)
+
+        def obj.eql?(o)
+          # It's undefined whether the impl does a[0].eql?(a[1]) or
+          # a[1].eql?(a[0]) so we taint both.
+          taint
+          o.taint
+          false
+        end
+
+        obj
+      end
+
+      a.uniq.should == a
+      a[0].tainted?.should == true
+      a[1].tainted?.should == true
+
+      a = Array.new(2) do
+        obj = mock('0')
+        obj.should_receive(:hash).at_least(1).and_return(0)
+
+        def obj.eql?(o)
+          # It's undefined whether the impl does a[0].eql?(a[1]) or
+          # a[1].eql?(a[0]) so we taint both.
+          taint
+          o.taint
+          true
+        end
+
+        obj
+      end
+
+      a.uniq.size.should == 1
+      a[0].tainted?.should == true
+      a[1].tainted?.should == true
+    end
   end
 
   it "compares elements based on the value returned from the block" do