Fix edge case with Grids, *s, and unconstrained layouts

[hartez]

Description of Change

In a layout like this one:

<VerticalStackLayout>
    <Grid RowDefinitions="*,*,*" VerticalOptions="Fill">
        <Border StrokeThickness="2" Stroke="LightBlue">
            <Label Text="Hello"></Label>
        </Border>
        <Border Grid.Row="1" Stroke="Orange" StrokeThickness="5">
            <Label Text="Hello" HeightRequest="25"></Label>
        </Border>
        <Border Grid.Row="2" Stroke="Red" StrokeThickness="10">
            <Label Text="Hello" HeightRequest="50"></Label>
        </Border>
    </Grid>
</VerticalStackLayout>

The Grid is taking the natural stacked size of the rows and trying to distribute the heights evenly (because of the * values), even though the VerticalOptions="Fill" shouldn't have any meaning in a vertically unconstrained layout. This ends up creating some weird layout results. The Grid should be ignoring the "Fill" in this situation and just leaving the rows at their originally measured heights. (A similar issue is present for columns in horizontally unconstrained layouts.)

This PR fixes the issue so the Grid behaves.

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[rmarinho]

/backport to net7.0

[github-actions]

Started backporting to net7.0: https://github.com/dotnet/maui/actions/runs/4599366267