Import Debian changes 1.2.18-1

network-manager-l2tp (1.2.18-1) unstable; urgency=medium

  * New upstream release.
  * Remove redundant -Wl,--as-needed as this is now default from Bullseye.
  * Updated copyright file.
  * Bump debhelper from old 11 to 12.
  * Set debhelper-compat version in Build-Depends.
  * Set upstream metadata fields: Bug-Database, Bug-Submit (from
    ./configure), Repository, Repository-Browse.