You could generate CAPTCHA images on your own server without connecting to any external servers as follows:
Draw text on an image that contains letters and/or numbers.
Use image processing to distort or camouflage the contents of the image to make it hard for OCR software to correctly read it.
Display the image on the website, possibly with a secondary ...
AdoptOpenJDK is now known as Adoptium, after moving to the Eclipse Foundation. But keep using the https://adoptopenjdk.net web site during their transition.
Adoptium offers many variations of Java implementations, for various CPU architectures, host OSes, and such. But none of them include the JavaFX libraries necessary to run a JavaFX (OpenJFX) app....