Open diffs plural for tasks upgrade notification

[sbatten]

Testing #122190

The notification for automatically upgrading from tasks v1 to v2 uses "diffs" instead of "diff". Will this ever open more than one diff editor? If not, I would expect "diff."

[alexr00]

When there is more than one workspace folder, then you could end up with multiple diffs if more than one folder has tasks. However, this can be smarter and use the correct word.