storcon_cli: don't clobber heatmap interval when setting eviction (#8722)

## Problem

This command is kind of a hack, used when we're migrating large tenants
and want to get their resident size down. It sets the tenant config to a
fixed value, which omitted heatmap_period, so caused secondaries to get
out of date.

## Summary of changes

- Set heatmap period to the same 300s default that we use elsewhere when
updating eviction settings

This is not as elegant as some general purpose partial modification of
the config, but it practically makes the command safer to use.

control_plane/storcon_cli/src/main.rs

@@ -622,6 +622,7 @@ async fn main() -> anyhow::Result<()> {
                                 threshold: threshold.into(),
                             },
                         )),
+                        heatmap_period: Some("300s".to_string()),
                         ..Default::default()
                     },
                 })