Fix a couple more CodeQL warnings (#17209)

Author: trask

smoke-tests/images/servlet/servlet-5.0/src/main/java/io/opentelemetry/smoketest/matrix/GreetingServlet.java

@@ -32,6 +32,7 @@ protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IO
     try (InputStream remoteInputStream = urlConnection.getInputStream()) {
       long bytesRead = transfer(remoteInputStream, buffer);
       String responseBody = buffer.toString("UTF-8");
+      resp.setContentType("text/plain");
       ServletOutputStream outputStream = resp.getOutputStream();
       outputStream.print(
           bytesRead + " bytes read by " + urlConnection.getClass().getName() + "\n" + responseBody);

smoke-tests/images/servlet/servlet-5.0/src/main/java/io/opentelemetry/smoketest/matrix/HeaderDumpingServlet.java

@@ -19,6 +19,7 @@ public class HeaderDumpingServlet extends HttpServlet {
 
   @Override
   protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
+    resp.setContentType("text/plain");
     PrintWriter response = resp.getWriter();
     Enumeration<String> headerNames = req.getHeaderNames();
     while (headerNames.hasMoreElements()) {