Import Debian changes 1.20.2-1

network-manager-l2tp (1.20.2-1) unstable; urgency=medium
.
  * New upstream release
  * Enable GTK 4 support (closes: #1009102)
  * Add Build-Depends libgtk-4-bin (>= 4.6.2)
    to ensure gtk4-builder-tool does not need a display
  * Bump libglib2.0-dev Build-Depends to (>= 2.34)
  * Bump libgtk-3-dev Build-Depends to (>= 3.16)
  * Enable all hardening build flags
  * Drop unnecessary network-manager version contraint
  * Change debian-branch from master to main in gbp.conf