chore: set LEAN_VERSION_IS_RELEASE to 1 for v4.28.0 release

Author: kim-em

src/CMakeLists.txt

@@ -12,7 +12,7 @@ project(LEAN CXX C)
 set(LEAN_VERSION_MAJOR 4)
 set(LEAN_VERSION_MINOR 28)
 set(LEAN_VERSION_PATCH 0)
-set(LEAN_VERSION_IS_RELEASE 0)  # This number is 1 in the release revision, and 0 otherwise.
+set(LEAN_VERSION_IS_RELEASE 1)  # 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'")
 set(LEAN_VERSION_STRING "${LEAN_VERSION_MAJOR}.${LEAN_VERSION_MINOR}.${LEAN_VERSION_PATCH}")
 if (LEAN_SPECIAL_VERSION_DESC)