Updated minimum cmake requirement down to 2.8.12

* As described in issue #3801 the upcoming cmake 3.19
  will not support cmake 2.6 any more
* This PR updates the mimimum required cmake version
  to 2.8.12, which will not give a warning with cmake 3.19
  but still compatible with MbedTLS support of RHEL/CentOS 7 LTS
* Adding ChangeLog.d/bugfix_PR3802.txt

Signed-off-by: Peter Toft <peter.toft@dirac.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5af4c81..822186d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -16,7 +16,7 @@
 #     will be removed as well as this note.
 #
 
-cmake_minimum_required(VERSION 2.6)
+cmake_minimum_required(VERSION 2.8.12)
 if(TEST_CPP)
     project("mbed TLS" C CXX)
 else()
diff --git a/ChangeLog.d/minimum_cmake_version_PR3802.txt b/ChangeLog.d/minimum_cmake_version_PR3802.txt
new file mode 100644
index 0000000..549f9b1
--- /dev/null
+++ b/ChangeLog.d/minimum_cmake_version_PR3802.txt
@@ -0,0 +1,3 @@
+Requirement changes
+* Update the minimum required CMake version to 2.8.12.
+* This silences a warning on CMake 3.19.0. #3801