https://lucene.apache.org/solr/guide/ needs to be updated on each release

[Colvin Cowie]

I've just noticed https://lucene.apache.org/solr/guide/ says 8.6 is the
latest release and doesn't mention 8.7

*Latest release*:

   - Solr 8.6 <https://lucene.apache.org/solr/guide/8_6/>