Update the install script for doppler to avoid the deprecated apt-key call

[MattHall]

Seeing error traces like this during installation:

1.605 Reading package lists...
1.842 W: https://dl.yarnpkg.com/debian/dists/stable/InRelease: Policy will
reject signature within a year, see --audit for details
1.844 ./install.sh: 36: apt-key: not found
1.949 ERROR: Feature "doppler CLI"
(ghcr.io/metcalfc/devcontainer-features/doppler) failed to install!

due to the deprecation of the apt-key command. Refactored the install script to
use the new recommended method from the doppler docs:

docs.doppler.com/docs/install-cli