[Lens] Get Heatmap out of experimental status

[flash1293]

The following items should get addressed before heatmap can be called non-experimental

elastic-charts related

• Value formatting doesn't apply to the legend labels. It's been fixed in the elastic-charts, should be part of the upcoming patch release.
• Intervals don't work 100% reliably (Heatmap time buckets should use value from timeScale elastic-charts#1165). We made a decision to disable xDomain until the issue is resolved on the elastic charts, so be aware the chart won't auto-extend with respect to the time range.
• Legends items might contain duplicated 0 (Heatmap legend with linear color scale contains duplicate 0  elastic-charts#1166)
• Legends is missing entries in some cases ([Heatmap] Legend occasionally missing some entires  elastic-charts#1191)
• Overlapping labels ([Heatmap] Rotate or hide overlapping labels on X-axis elastic-charts#1168)
• Overlay mask for brushing over categorical data isn't rendered (Heatmap brushing action for categorical data  elastic-charts#1170)
• Brushing action over the numeric categorical data crashes the browser's window (Brushing action crashes the the browser's window elastic-charts#1171)
• Scrollbar appears on resizing browser window [Lens] Get Heatmap out of experimental status #102036
• Heatmap axis label controls [Heatmap] Add axes titles elastic-charts#1221

require changes on the Lens side

• Value label controls in heatmap ([Lens] Add Heatmap option to show cell values #105410)
• [Lens] Heatmap problem with colouring cells with value equal to the min value #117512

later

• Time range brushing and terms selecting doesn't work at the same time [Lens][uiActions] Allow brushing and filtering at the same time using combined context #103062
• Axis ordering. The heatmap chart supports xSortPredicate and ySortPredicate (at the moment using dataIndex for both). Axis configuration in Lens should allow configuring the sorting function.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[stratoula]

Heatmap is on GA, we can close it!