[DataGridPremium] Use the aggregated value on tree data real groups

[flaviendelangle]

Fixes #5923

Two fixes for the aggregation used with tree data:

• When using a tree of depth < 2, do not aggregate a group based on both a child group and this child group childrens
• Correctly render the aggregated value instead of the provided value on data group like told in the doc

Demo: https://codesandbox.io/s/nameless-worker-oox3gk?file=/demo.tsx

[mui-bot]

These are the results for the performance tests:

Test case	Unit	Min	Max	Median	Mean	σ
Filter 100k rows	ms	338.1	605.8	447.5	478.94	103.329
Sort 100k rows	ms	520.8	836.8	801.2	754.5	118.497
Select 100k rows	ms	135.6	282.1	218.6	216.24	47.12
Deselect 100k rows	ms	139.3	317.6	189.1	223.92	69.952

Generated by 🚫 dangerJS against c0fea18