[Java] Fix serialized name of discriminator in JSON.mustache

Created by: sebastien-rosset

This PR has a fix for #4671 (closed). The JSON.mustache template should use "propertyBaseName" tag instead of "propertyName". The "propertyBaseName" tag is the name of the serialized property, whereas "propertyName" is the Java variable name.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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modules/openapi-generator/src/main/resources/Java/JSON.mustache

@@ -75,7 +75,7 @@ public class JSON {
                         {{/mappedModels}}
                         classByDiscriminatorValue.put("{{classname}}".toUpperCase(Locale.ROOT), {{classname}}.class);
                         return getClassByDiscriminator(classByDiscriminatorValue,
-                                getDiscriminatorValue(readElement, "{{{propertyName}}}"));
+                                getDiscriminatorValue(readElement, "{{{propertyBaseName}}}"));
                     }
           })
         {{/discriminator}}