Bump vaadin-bom from 8.4.2 to 8.12.3 in /vaadin8-activiti#29

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dependabot/maven/vaadin8-activiti/com.vaadin-vaadin-bom-8.12.3cherkavi/java-web:dependabot/maven/vaadin8-activiti/com.vaadin-vaadin-bom-8.12.3Copy head branch name to clipboard