Upgrade the default version of Solr

[HoustonPutman]

Currently the Solr Operator will choose the Solr version (tag) 7.7.0 for Solr Clouds and Prometheus Exporters if none is provided. This is extremely out of date, and should not be recommended for use and especially not used as the default.

We should probably update this more often, since the default version is written to the SolrCloud resource when it is created, so changing the default will not change any already-running Solr Clouds. It will just change the behavior for new clouds that lack a Solr version.