Larger close button in fluent dialog header template

[drewnoakes]

(Pulled from dotnet/aspire#5200 (comment))

@drewnoakes wrote:
Can we make the "close" button bigger, so it's easier to see and press in a mobile device form factor?


Ignore the filter button at the bottom in these screenshots.

@adamint wrote:
This comes from the default fluent dialog header template, I would suggest instead trying to get this size increase on fluentui-blazor for general usability. @vnbaaij have you considered increasing the size of the close icon? It seems relatively small compared to the rest of the header content height.

[drewnoakes]

Thanks @vnbaaij!