...or make a new one
You'll get commands you can copy & paste into Minecraft to generate your QR code out of blocks. Works in local or multiplayer/realms (as long as you are op), tested on Minecraft Java Edition.
Open Minecraft in a window on this computer (you can toggle fullscreen with F11).
Go to the spot where you'd like to put the QR code. Note that things nearby may be destroyed if the QR code overlaps them, so try this out somewhere that isn't near anything important!
Open the chat by pressing T. Copy and paste in the following:
/setblock ^ ^1 ^ minecraft:command_block
Note that for this to work you must enable cheats.
Create a button next to the command block. Your screen should now look something like this.
Click the code below to copy it. copied!
Right-click on your command block, then paste (ctrl-v on Windows, ⌘v on mac).
You should now see something like this:
Click "done" to close the command block editor, then press the button you created.
The QR code is not yet finished. Read on.
If all went well something like this should have happened:
Your QR code is finished!
Try pointing the camera app on your phone at it, and the link that you entered should pop up.
Gather your friends around to witness the magnificence of your creation!
Minimizing command count...
Also see qrpicture.com if you'd like to create QR codes out of pictures for use outside of Minecraft.