chore: begin development cycle for v4.22.0 (#8642)

This PR bumps the version number of the Lean project to 4.22.0, since
v4.21.0 is now in the release candidate stage.

Author: Vierkantor

src/CMakeLists.txt

@@ -10,7 +10,7 @@ endif()
 include(ExternalProject)
 project(LEAN CXX C)
 set(LEAN_VERSION_MAJOR 4)
-set(LEAN_VERSION_MINOR 21)
+set(LEAN_VERSION_MINOR 22)
 set(LEAN_VERSION_PATCH 0)
 set(LEAN_VERSION_IS_RELEASE 0)  # This number is 1 in the release revision, and 0 otherwise.
 set(LEAN_SPECIAL_VERSION_DESC "" CACHE STRING "Additional version description like 'nightly-2018-03-11'")