Make UnifiedHighlighter the default

I think the UnifiedHighlighter should be the default in 8.0. It's faster and more accurate than alternatives.

The original highlighter however has some benefits:

• Different passage/snippet delineation options; somewhat more flexible. Though no i18n BreakIterator based one.
• Seems to handle some "special" Queries and/or QueryParsers by default better – namely SurroundQParser. Though SOLR-12895 will address this UH issue.
• Considers boosts in the query when computing a passage score
• hl.alternateField, hl.maxAlternateFieldLength, hl.highlightAlternate options. Instead the UH has hl.defaultSummary boolean

See https://builds.apache.org/job/Solr-reference-guide-master/javadoc/highlighting.html