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/clear_spec.rb

@@ -20,24 +20,28 @@ describe "Array#clear" do
     a.size.should == 0
   end
 
-  it "keeps tainted status" do
-    a = [1]
-    a.taint
-    a.tainted?.should be_true
-    a.clear
-    a.tainted?.should be_true
+  ruby_version_is ''...'2.7' do
+    it "keeps tainted status" do
+      a = [1]
+      a.taint
+      a.tainted?.should be_true
+      a.clear
+      a.tainted?.should be_true
+    end
   end
 
   it "does not accept any arguments" do
     -> { [1].clear(true) }.should raise_error(ArgumentError)
   end
 
-  it "keeps untrusted status" do
-    a = [1]
-    a.untrust
-    a.untrusted?.should be_true
-    a.clear
-    a.untrusted?.should be_true
+  ruby_version_is ''...'2.7' do
+    it "keeps untrusted status" do
+      a = [1]
+      a.untrust
+      a.untrusted?.should be_true
+      a.clear
+      a.untrusted?.should be_true
+    end
   end
 
   it "raises a #{frozen_error_class} on a frozen array" do