[java] Fix hashCode and equals for additionalProperties and generateAliasAsModel

[jpfinne]

Fix compilation error for hashcode #20795

Equals also uses additionalProperties if present.

@NilsDammenhayn
The generated code still uses the "new" way of 7.12.0

private Map<String, Object> additionalProperties;

instead of

extends HashMap<String, Object>

This PR only fixes the compilation error for resttemplate, webclient and feign

Microprofile still uses the extends HashMap

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