[Java][Feign] convert expirationTimeSeconds to milliseconds

Created by: kmaginot

Hi @wing328 @bbdouglas @sreeshas, @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608, this is my proposed fix for https://github.com/OpenAPITools/openapi-generator/issues/13339

The Java OAuth RequestInterceptor implementation for Feign currently verifies if a previously obtained access token needs to be renewed by checking if System.currentTimeMillis() is greater than expirationTimeSeconds. This causes a new access token to be requested for each interaction. Converting expirationTimeSeconds to milliseconds should fix this.

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