MDEV-26438 cmake < 3.6.0 produced RPMs with invalid names

replaced CPACK_RPM_PACKAGE_VERSION with SERVER_VERSION.
CPACK_* variables are empty and can't be used until INCLUDE(CPack) is
called.
SERVER_VERSION is the safest option because other variables may be
overwritten from submodules

cmake/cpack_rpm.cmake

@@ -35,7 +35,7 @@ SET(CPACK_COMPONENTS_ALL Server ManPagesServer IniFiles Server_Scripts
 SET(CPACK_RPM_PACKAGE_NAME ${CPACK_PACKAGE_NAME})
 SET(CPACK_RPM_PACKAGE_VERSION ${CPACK_PACKAGE_VERSION})
 IF(CMAKE_VERSION VERSION_LESS "3.6.0")
-  SET(CPACK_PACKAGE_FILE_NAME "${CPACK_RPM_PACKAGE_NAME}-${CPACK_RPM_PACKAGE_VERSION}-${RPM}-${CMAKE_SYSTEM_PROCESSOR}")
+  SET(CPACK_PACKAGE_FILE_NAME "${CPACK_RPM_PACKAGE_NAME}-${SERVER_VERSION}-${RPM}-${CMAKE_SYSTEM_PROCESSOR}")
 ELSE()
   SET(CPACK_RPM_FILE_NAME "RPM-DEFAULT")
   OPTION(CPACK_RPM_DEBUGINFO_PACKAGE "" ON)