DataGrid: Group header column width fix

[tesar-tech]

Description

Closes #5804

The issue arises because the header group column inherits its width from the first header column with the specified HeaderGroupCaption. In my opinion, setting a width on the header group isn’t necessary, at least from what I can foresee.

The simplest solution is to set Width to null. Simply removing the width from the Razor file (line 42 in DataGrid) won’t be effective, as the width is also applied through Style property.