Update boost build env

[hendrikmuhs]

after updating boost in #314, this PR fixes some issues:

• newer versions of boost require linking the process library, see Compiling Boost.Process as header-only fails  boostorg/process#390
  • older versions don't ship boost process and the upcoming 1.89 might go back to header-only
• fix linking on macos for newer boost and remove pinning 1.85
  • reason: upstream brew package removed boost_thread-mt, now there is only boost_thread (-mt is now default), same as on Linux
  • Mac users that compile from source need to update boost
• fix cmake deprecation warning
  • only applies to very old boost versions which aren't supported anyway, raised the minimum required versions to 1.80 for boost process v2
  • update ubuntu version for rust workflow to get newer boost there
    • rust bindings don't support keyvi index, that's why the newer boost wasn't necessary