Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
fmt 8 checks format strings at compile time, and requires that
non-compile-time format strings be wrapped with fmt::runtime().
Do that, and to allow coexistence with fmt 7, supply our own
do-nothing version of fmt::runtime() if needed. Strictly speaking
we shouldn't be introducing names into the fmt namespace, but this
is transitional only.
Closes#9640
