Solr search system: Upgrade to v7

The work is to upgrade the search software used for the Historic England main website from our currently installed version 5 to the most recent version 7.

This work is required to improve the scalability and resilience of the system; to prepare for IMT moving hosting from Claranet to Azure; and as a precursor to incorporating the Archives database of approx. 12m records. Once this work is complete we can index the Archive data and close 3 satellite websites currently used by the Archive.

As part of this work we would also renew our current support which includes 3 system health checks throughout the year.