[build] Fix CMake configure warnings

FetchContent requires CMake 3.11 (released Nov 30, 2018).

Fixed this warning:
```
CMake Warning (dev) at /usr/share/cmake/Modules/FetchContent.cmake:1316 (message):
  The DOWNLOAD_EXTRACT_TIMESTAMP option was not given and policy CMP0135 is
  not set.  The policy's OLD behavior will be used.  When using a URL
  download, the timestamps of extracted files should preferably be that of
  the time of extraction, otherwise code that depends on the extracted
  contents might not be rebuilt if the URL changes.  The OLD behavior
  preserves the timestamps from the archive instead, but this is usually not
  what you want.  Update your project to the NEW behavior or specify the
  DOWNLOAD_EXTRACT_TIMESTAMP option with a value of true to avoid this
  robustness issue.
Call Stack (most recent call first):
  imgui/CMakeLists.txt:23 (FetchContent_Declare)
```

CMakeLists.txt

@@ -11,10 +11,13 @@ if ("${CMAKE_HOST_SYSTEM_NAME}" STREQUAL "Windows")
     set(CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION 10.0.18362.0 CACHE STRING INTERNAL FORCE)
 endif()
 
-cmake_minimum_required(VERSION 3.3.0)
+cmake_minimum_required(VERSION 3.11)
 project(allwpilib)
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
+# Make timestamps of extracted files from FetchContent the time of extraction
+cmake_policy(SET CMP0135 NEW)
+
 message(STATUS "Platform version: ${CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION}")
 
 set(WPILIB_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})

README-CMAKE.md

@@ -93,7 +93,7 @@ Using the libraries from C++ is the easiest way to use the built libraries.
 To do so, create a new folder to contain your project. Add the following code below to a `CMakeLists.txt` file in that directory.
 
 ```
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.11)
 project(vision_app) # Project Name Here
 
 find_package(wpilib REQUIRED)

cmake/toolchains/gnu.toolchain.cmake

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.3.0)
+cmake_minimum_required(VERSION 3.11)
 
 # load settings in case of "try compile"
 set(TOOLCHAIN_CONFIG_FILE "${WPILIB_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/toolchain.config.cmake")