CPack/NSIS: Fix silent NSIS installs

The installer should not put up a message box to confirm uninstall of a
previous version for silent installs. Furthermore, when the uninstaller
is invoked, do not show its confirmation dialog because either (a) the
installer already asked the user or (b) the installer is running in
silent mode.

Closes: #16328

Modules/NSIS.template.in

@@ -905,7 +905,7 @@ Function .onInit
 
   MessageBox MB_YESNOCANCEL|MB_ICONEXCLAMATION \
   "@CPACK_NSIS_PACKAGE_NAME@ is already installed. $\n$\nDo you want to uninstall the old version before installing the new one?" \
-  IDYES uninst IDNO inst
+  /SD IDYES IDYES uninst IDNO inst
   Abort
 
 ;Run the uninstaller
@@ -913,7 +913,7 @@ uninst:
   ClearErrors
   StrLen $2 "\Uninstall.exe"
   StrCpy $3 $0 -$2 # remove "\Uninstall.exe" from UninstallString to get path
-  ExecWait '"$0" _?=$3' ;Do not copy the uninstaller to a temp file
+  ExecWait '"$0" /S _?=$3' ;Do not copy the uninstaller to a temp file
 
   IfErrors uninst_failed inst
 uninst_failed: