[DOCS] 8.3 screenshot updates for Cases

[lcawl]

Description

https://www.elastic.co/guide/en/security/master/cases-overview.html contains screenshots which are affected by new functionality in 8.3

For example:

• cases-home-page.png is affected by the new "Average time to close" metric added in [Cases] Show the Average time to close stat in the all cases page kibana#131909 and by the removal of text from the "Reporters" column in [Cases] Improve reporter column in the cases table kibana#132200
• cases-ui-open.png, cases-summary.png, and add-vis-to-case.gif are affected by the new "Severity" property added in [Cases] severity field in the cases list and allow to filter by it kibana#131694

[nastasha-solomon]

Will also need to doc the new metrics shown on the Cases home page. See convo here for more info about the metric definition.

[nastasha-solomon]

Merged #2050