Bump vaadin-bom from 8.9.0 to 8.12.3 in /filedroptarget-and-selector-demo#2

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dependabot/maven/filedroptarget-and-selector-demo/com.vaadin-vaadin-bom-8.12.3gerald24/FileDropTargetAndSelector:dependabot/maven/filedroptarget-and-selector-demo/com.vaadin-vaadin-bom-8.12.3Copy head branch name to clipboard