Use EXISTING_PROPERTY for JavaSpring as well

[keenanpepper]

This fixes issue #3796 for JavaSpring. It's a very straightfoward extension
of #5120 for the JavaSpring generator (that PR was just for the Java generator).

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[keenanpepper]

@bkabrda @jimschubert

[jimschubert]

Thanks for adding this consistency across the Java generators supporting discriminators.

[wing328]

@keenanpepper thanks for the PR, which has been included in the 4.3.0 release: https://twitter.com/oas_generator/status/1243455743937789952