Migrate REUSE metadata from deprecated .reuse/dep5 to REUSE.toml

reuse==6.2.0 (pinned in cmake/requirements/requirements-reuse.txt) warns
"'.reuse/dep5' is deprecated. You are recommended to instead use
REUSE.toml. Use `reuse convert-dep5` to convert." on every
ci_reuse_compliance run. A later reuse release that drops DEP5 support
would break that CI job outright.

Generate REUSE.toml from the 9 Files: stanzas with `reuse convert-dep5`
and remove .reuse/dep5. `reuse lint` still reports full compliance with
spec 3.3 (1268/1268 files with copyright and license information,
unchanged), and `reuse spdx` produces the same per-file copyright and
license entries as before the conversion.

Update the three places that referenced the old file: FILES.md now
documents REUSE.toml with a spec 3.3 link, README.md's REUSE section
names REUSE.toml, and the comment in gen_hedley_undef_check.cmake now
refers to REUSE.toml's blanket path = "**" rule.

Overlaps #4663, which adds an entry to .reuse/dep5; that PR will need to
move its entry into REUSE.toml when rebased on this.

#5716 item 4

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

cmake/scripts/gen_hedley_undef_check.cmake

@@ -64,7 +64,7 @@ if(MODE STREQUAL "undef")
     # recipe is self-contained and its output is byte-stable across reruns.
     # The embedded SPDX tags below are part of the *generated* file's
     # content, not a REUSE header for this .cmake script itself (which is
-    # already covered by the blanket "Files: *" rule in .reuse/dep5) -- keep
+    # already covered by the blanket path = "**" rule in REUSE.toml) -- keep
     # them wrapped in REUSE-IgnoreStart/End so `reuse lint` does not try to
     # parse "MIT\n")" as this file's own SPDX-License-Identifier value.
     # REUSE-IgnoreStart