Node 6 in Azure Pipelines is end-of-life

[fntc]

Running a pipeline with this Task shows the following warning:

##[warning]Task 'Code Signing' version 2 (codesigning@2) is dependent on a Node version (6) that is end-of-life. Contact the extension owner for an updated version of the task. Task maintainers should review Node upgrade guidance: https://aka.ms/node-runner-guidance

Proposed fix: Update used node version