Incorrect frontend payload when updating Heatmap visualization

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Go into Explore
2. Select any value you like
3. Select Heatmap Visualization
4. Change Y Axis Domain to fixed values, example: 0, 100
5. Apply - Works as expected
6. Save as a Dashboard Cell
7. Get error


```
Failed to add cell: json: cannot unmarshal string into Go struct field HeatmapViewProperties.yDomain of type float64
```

In the request payload `properties.yDomain` has value `["0", "100"]` instead of `[0, 100]`

__Expected behavior:__
Correctly save as a dashboard cell

__Environment info:__

Running docker pinned version `influxdb:2.0.6-alpine`

Start by reproducing the failure through Explore with the Heatmap visualization and saving it as a dashboard cell. Inspect the frontend path that builds the dashboard-cell request and verify that properties.yDomain is sent as numeric values; done when the cell saves successfully without the unmarshalling error.

Written by the indexing model from the issue text.