cmake: remove Boost::system linkage for boost 1.89+

cmake: remove Boost::system linkage for boost 1.89+

In boost 1.89, the stub compiled library for Boost.System was removed.
According to the boost 1.89 release notes: "The stub compiled library
has been removed; System has been header-only since release 1.69.",
See https://www.boost.org/releases/1.89.0/ .

This change removes or replaces Boost::system linkage throughout the
codebase:

1. Removed "system" from BOOST_COMPONENTS in the root CMakeLists.txt,
   as the library no longer exists in boost 1.89+.

2. Where Boost::system was the only linked boost library, replaced it
   with Boost::boost (the header-only meta-target) to maintain access
   to Boost.System's header-only functionality.

3. Where other boost libraries were already linked (e.g., Boost::thread,
   Boost::regex, Boost::filesystem), removed Boost::system entirely, as
   the header-only System library dependency is automatically satisfied
   by other boost components.

This is not a breaking change since the minimum required boost version
for this project is 1.87, where Boost.System was already header-only
(only the stub compiled library remained for compatibility).

Signed-off-by: Kefu Chai <kchai@redhat.com>