Update Checkstyle DTD version from 1.0 to 1.2

Author: garydgregory

src/conf/checkstyle-suppressions.xml

@@ -1,5 +1,4 @@
 <?xml version="1.0" encoding="UTF-8"?>
-
 <!--
 Licensed to the Apache Software Foundation (ASF) under one or more
 contributor license agreements.  See the NOTICE file distributed with
@@ -16,11 +15,9 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 -->
-
 <!DOCTYPE suppressions PUBLIC
-    "-//Checkstyle//DTD SuppressionFilter Configuration 1.0//EN"
-    "https://checkstyle.org/dtds/suppressions_1_0.dtd">
-
+    "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
+    "https://checkstyle.org/dtds/suppressions_1_2.dtd">
 <suppressions>
     <suppress checks="Header" files="LICENSE.txt"/>
     <suppress checks="Header" files="NOTICE.txt"/>