util: Update tinyformat to upstream

Summary:

Last update was in 2017.
Updates tinyformat to upstream commit 705e3f4e1de922069bf715746d35bd2364b1f98f.
Re-apply bitcoin specific changes.
No changes that affect our use, as far as I can see, but this gets rid
of the gcc -Wimplicit-fallthrough warnings, at least.

This is a backport of Core PR17682

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8307