Release-ready candidate for upstream 1.4.2+dfsg

• New upstream version 1.4.2+dfsg
  • Refresh and drop applied by upstream patches
  • Add patch aliasing two renamed but unchanged symbols to maintain backwards compatibility
  • Build with ZSTD_LEGACY_MULTITHREADED_API=1 to maintain backwards compatibility
  • Update symbols file for libzstd1
  • Call dh_makeshlibs with -V 'libzstd1 (>= 1.4.0)', since this version introduced new public symbols
  • Bump debhelper compat level to 12.
  • Bump Standards-Version to 4.4.0, no changes needed.