Switch from BMP to PNG for graphical MSI installer assets (#9606)

This breaks rendering of images for Windows 7 and Server 2008R2,
but it's required for accessibility purposes so that the High
Contrast Black theme renders text correctly for accessibility.
(Unfortunately, BMP does not support transparency.)

assets/Product.wxs

@@ -41,9 +41,9 @@
     <!-- Set properties for add/remove programs -->
     <Property Id="ARPHELPLINK" Value="$(var.InfoURL)" />
     <!-- custom images -->
-    <WixVariable Id="WixUIBannerBmp" Value="assets\WixUIBannerBmp.bmp" />
-    <WixVariable Id="WixUIDialogBmp" Value="assets\WixUIDialogBmp.bmp" />
-    <WixVariable Id="WixUIExclamationIco" Value="assets\WixUIInfoIco.bmp" />
+    <WixVariable Id="WixUIBannerBmp" Value="assets\WixUIBannerBmp.png" />
+    <WixVariable Id="WixUIDialogBmp" Value="assets\WixUIDialogBmp.png" />
+    <WixVariable Id="WixUIExclamationIco" Value="assets\WixUIInfoIco.png" />
     <WixVariable Id="WixUIInfoIco" Value="assets\ps_black_32x32.ico" />
     <Property Id="WIXUI_EXITDIALOGOPTIONALCHECKBOXTEXT" Value="Open $(env.ProductName)" />
     <!-- Default value of Checkbox of starting PowerShell after installation -->