Change Retention of `dbo.all_server_volatile_info_history` to 15 Days

[imajaydwivedi]

There are scenarios where we need to check historical metrics to figure out max/min/average/pencentile etc for a metrics within x days.

For example, get max memory utilization for last 7 days.

Need to change Retention of dbo.all_server_volatile_info_history to 15 Days in table dbo.purge_table.