build: move gtest into cmake and up cmake min version

cmake minimum required version is now 3.25.

Instead of loading gtest via git submodule, it is now loaded
in the cmake files.

gtest version upped to v1.13.0.

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "libs/gtest"]
-	path = libs/gtest
-	url = https://github.com/google/googletest.git

CHANGELOG.md

@@ -16,6 +16,8 @@ maddy uses [semver versioning](https://semver.org/).
 
 * ![**ADDED**](https://img.shields.io/badge/-ADDED-%23099) Added Changelog
 * ![**ADDED**](https://img.shields.io/badge/-ADDED-%23099) Added contribution guideline
+* ![**CHANGED**](https://img.shields.io/badge/-CHANGED-%23e90) updated cmake minimum required version to 3.25
+* ![**CHANGED**](https://img.shields.io/badge/-CHANGED-%23e90) gtest is now loaded via cmake and not a git submodule any longer - updated gtest version to 1.13.0
 * ?
 
 ## version 1.1.2 2020-10-04

CMakeLists.txt

@@ -1,7 +1,7 @@
 # This project is licensed under the MIT license. For more information see the
 # LICENSE file.
 
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.25)
 
 project(maddy)
 
@@ -54,8 +54,6 @@ endif()
 
 # ------------------------------------------------------------------------------
 
-set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/libs/gtest/googlemock)
 add_subdirectory(libs)
 
 # ------------------------------------------------------------------------------

libs/CMakeLists.txt

@@ -1,14 +1,11 @@
 # This project is licensed under the MIT license. For more information see the
 # LICENSE file.
 
-cmake_minimum_required(VERSION 2.8)
+include(FetchContent)
 
-set(LIBS_INCLUDE_DIRS
-# -- googletest / -mock --------------------------------------------------------
-  ${CMAKE_CURRENT_SOURCE_DIR}/gtest/googletest/include
-  ${CMAKE_CURRENT_SOURCE_DIR}/gtest/googlemock/include
-# ------------------------------------------------------------------------------
-PARENT_SCOPE)
-
-set(LIBS_SRC_FILES
-PARENT_SCOPE)
+FetchContent_Declare(
+  googletest
+  GIT_REPOSITORY https://github.com/google/googletest.git
+  GIT_TAG        b796f7d44681514f58a683a3a71ff17c94edb0c1 # v1.13.0
+)
+FetchContent_MakeAvailable(googletest)