cmake: pin seastar to c++20

The combination of libstdc++ >= 13, c++ >= 23 and clang 16 has this bug
that caused compilation issues on ubuntu:
 * llvm/llvm-project#61415

This does not cause any compilation errors on fedora because fedora has
a patch version higher gcc and libstdc++ version.

Seastar recently upgraded to using c++23, so this is why we are seeing
this error now.

cmake/dependencies.cmake

@@ -37,11 +37,13 @@ fetch_dep(fmt
 # the add_subdirectory method of using Seastar.
 set(Seastar_TESTING ON CACHE BOOL "" FORCE)
 set(Seastar_API_LEVEL 6 CACHE STRING "" FORCE)
+set(CMAKE_CXX_STANDARD 20)
 set(Seastar_CXX_FLAGS -Wno-error)
 fetch_dep(seastar
   REPO https://github.com/redpanda-data/seastar.git
   TAG v24.2.x
   PATCH_COMMAND sed -i "s/add_subdirectory (tests/# add_subdirectory (tests/g" CMakeLists.txt)
+set(CMAKE_CXX_STANDARD 20)
 
 fetch_dep(avro
   REPO https://github.com/redpanda-data/avro