fix apt-key deprecation ( Key is stored in legacy trusted.gpg keyring warning/error )

fix-apt-key-legacy-warning.txt

Fix apt warnings about keys "stored in legacy trusted.gpg keyring" 
Add a key to /etc/apt/trusted.gpg.d/  
For instance with sysdig , the key is in ascii armored format and available at  https://download.sysdig.com/ 
this simple command will immediately get apt to stop complaining:

wget -qO- https://download.sysdig.com/DRAIOS-GPG-KEY.public | sudo tee /etc/apt/trusted.gpg.d/sysdig.asc

If the file is in "GPG key public ring" binary OpenPGP format then use .gpg extension. 

Also See note in apt-key(8) :
" Instead of placing keys into the /etc/apt/trusted.gpg.d directory, 
   you can place them anywhere on your filesystem by using the Signed-By 
   option in your sources.list and pointing to the filename of the key. "