Add deprecation warnings for policies CMP0072 and below

The OLD behaviors of all policies are deprecated, but only by
documentation.  Add an explicit deprecation diagnostic for policies
introduced in CMake 3.11 and below to encourage projects to port away
from setting policies to OLD.

Help/release/dev/deprecate-policy-old.rst

@@ -0,0 +1,8 @@
+deprecate-policy-old
+--------------------
+
+* An explicit deprecation diagnostic was added for policy ``CMP0071``
+  (``CMP0071`` and below were already deprecated).
+  The :manual:`cmake-policies(7)` manual explains that the OLD behaviors
+  of all policies are deprecated and that projects should port to the
+  NEW behaviors.

Source/cmMakefile.cxx

@@ -4554,7 +4554,7 @@ bool cmMakefile::SetPolicy(cmPolicies::PolicyID id,
 
   // Deprecate old policies, especially those that require a lot
   // of code to maintain the old behavior.
-  if (status == cmPolicies::OLD && id <= cmPolicies::CMP0071 &&
+  if (status == cmPolicies::OLD && id <= cmPolicies::CMP0072 &&
       !(this->GetCMakeInstance()->GetIsInTryCompile() &&
         (
           // Policies set by cmCoreTryCompile::TryCompileCode.

Tests/RunCMake/FindOpenGL/CMP0072-OLD-stderr.txt

@@ -0,0 +1,10 @@
+^CMake Deprecation Warning at CMP0072-OLD.cmake:1 \(cmake_policy\):
+  The OLD behavior for policy CMP0072 will be removed from a future version
+  of CMake.
+
+  The cmake-policies\(7\) manual explains that the OLD behaviors of all
+  policies are deprecated and that a policy should be set to OLD only under
+  specific short-term circumstances.  Projects should be ported to the NEW
+  behavior and not rely on setting a policy to OLD.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)$