String Freeze Exception Request for Spectacle MR 300

[Noah Davis]

This patch changes the format of filename placeholders. The only
localizable changes are in the documentation and description for the
number sequence placeholder. It's not essential that it be merged
before 24.02 is branched, but merging it would allow me to close a bug
and work on some other patches that would let me close another bug
report.

https://invent.kde.org/graphics/spectacle/-/merge_requests/300