You can create a new Automator.app Quick Action document and add a "Run AppleScript" action to your workflow.
Then you would paste this following AppleScript code into the "Run AppleScript" action.
tell application "Preview" to close window 1 saving no
You can also use this following AppleScript code, instead, if you prefer to close all of the Preview documents at the same time without any dialogs popping up
tell application "Preview" to close windows saving no
Then you could save the Automator file as "Preview Close No Save". Once that file is saved you can now assign it a keyboard shortcut in System Preferences. Now anytime you are using Preview app, any time you use your new assigned keyboard shortcut, it should automatically close the visible document without popping up any dialogs.
If for some reason your new keyboard shortcut does not work, it may be because it conflicts with another keyboard shortcut already assigned. In that case try assigning it a new keyboard shortcut.