ignore unknown fields in index-v1 JSON data

Having Jackson set to ignore unknown fields in the incoming JSON data,
instead of throwing an Exception, means that we can add any fields to the
JSON without having to rev the index version, and older clients will still
parse it fine.  This is basically the same as in index.xml.
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f