Update Pico SDK dependence to 2.1.0 (#189)

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 318924a..8008d0e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,8 +19,8 @@
 endif ()
 include(${PICO_SDK_PATH}/pico_sdk_version.cmake)
 
-if (PICO_SDK_VERSION_STRING VERSION_LESS "2.0.0")
-    message(FATAL_ERROR "Raspberry Pi Pico SDK version 2.0.0 (or later) required. Your version is ${PICO_SDK_VERSION_STRING}")
+if (PICO_SDK_VERSION_STRING VERSION_LESS "2.1.0")
+    message(FATAL_ERROR "Raspberry Pi Pico SDK version 2.1.0 (or later) required. Your version is ${PICO_SDK_VERSION_STRING}")
 endif()
 
 # Set PICOTOOL_CODE_OTP to compile OTP definitions in - otherwise, they are included from JSON