feat docs: start 2.8-rc

Tests: протестировано CI
commit_hash:a0f972734212d616a59c35814255da8299eb7128

Author: apolukhin

cmake/GetUserverVersion.cmake

@@ -23,7 +23,7 @@ else()
 endif()
 
 set(USERVER_MAJOR_VERSION 2)
-set(USERVER_MINOR_VERSION 7)
+set(USERVER_MINOR_VERSION 8-rc)
 
 set(USERVER_VERSION "${USERVER_MAJOR_VERSION}.${USERVER_MINOR_VERSION}")
 string(REPLACE "-" "_" USERVER_VERSION_STR "${USERVER_VERSION}")