Build libcodec2_client with gnu++20

Bug: http://b/343951602

Explicitly set cpp_std for libcodec2_client to gnu++20.  The default
inherited from libcodec2-impl-defaults sets it to gnu++17 which causes a
segfault when mixing global std::string symbols built with gnu++17 and
gnu++20.  Once the  libcodec2-impl-defaults setting to gnu++17 is
removed, this can be cleaned up.

Test: lunch aosp_cf_x86_64_phone-userdebug && m && acloud create with clang-r522817
Change-Id: I72991835a29ef923284e81fdf3c1dd28a6b99663