doc: cmake package: remove stale cmake version lines

This page is meant to document the behavior of the Zephyr cmake
package. However, its example CMake snippets contain some stale
references to a minimum version (3.13.1, now 3.20).

These lines are a bit of a distraction anyway, so just remove them.
They matter in general, but they don't matter where the Zephyr cmake
package is concerned.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
diff --git a/doc/guides/zephyr_cmake_package.rst b/doc/guides/zephyr_cmake_package.rst
index 995754f..abc818f 100644
--- a/doc/guides/zephyr_cmake_package.rst
+++ b/doc/guides/zephyr_cmake_package.rst
@@ -248,7 +248,6 @@
 
   .. code-block:: cmake
 
-     cmake_minimum_required(VERSION 3.13.1)
      find_package(Zephyr 2.2.0)
      project(app)
 
@@ -283,7 +282,6 @@
 
 .. code-block:: cmake
 
-   cmake_minimum_required(VERSION 3.13.1)
    find_package(Zephyr 2.a)
    project(app)
 
@@ -295,7 +293,6 @@
 
 .. code-block:: cmake
 
-   cmake_minimum_required(VERSION 3.13.1)
    find_package(Zephyr 2.a EXACT)
    project(app)
 
@@ -303,7 +300,6 @@
 
 .. code-block:: cmake
 
-   cmake_minimum_required(VERSION 3.13.1)
    find_package(Zephyr 2.z)
    project(app)
 
@@ -363,8 +359,6 @@
 
 .. code-block:: cmake
 
-   cmake_minimum_required(VERSION 3.13.1)
-
    set(ZEPHYR_PREFER "zephyr-custom" "zephyr-vendor")
    find_package(Zephyr)
 
@@ -382,8 +376,6 @@
 
 .. code-block:: cmake
 
-   cmake_minimum_required(VERSION 3.13.1)
-
    set(ZEPHYR_PREFER "zephyr-test")
    find_package(Zephyr)