warn about unused UnifiedSolrHighlighter configuration

[cpoerschke]

No JIRA issue as yet but would create one if this seem useful.

The default config has a comment

<!-- note: the hl.method=unified highlighter is not configured here; it's completely configured
    via parameters.  The below configuration supports hl.method=original and fastVector. -->

i.e. https://github.com/apache/solr/blob/branch_9_0/solr/server/solr/configsets/_default/conf/solrconfig.xml#L849-L850 but it still could be useful to flag unused configuration in the logs?

[epugh]

Instead of adding more complexity, could we just configure the unified highlighter here and get rid of the others?

I like the ida of identifying unused configurations, but maybe that is something that needs to be done globablly (and I don't have any idea how you would do that!). I see a LOT of solrconfigs.xml that all need to go on a diet and have tons of unused stuff ;-).

I wish we shipped a nice stripped down solrconfig/schema that would encourage people to add to it, versus the filled with assumptions that is _default or the kitchen sink that is techproducts.

[dsmiley]

Unless I'm missing something (maybe I looked too quickly at this PR), the change seems invalid to me. If someone registers highlighters, they can be chosen at runtime via hl.method.

BTW total +1 to what Eric said. That's another issue though; and JIRA worthy.

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