chore: add changelog for 3.16.0 #27266

Job	Run time
Nix (macos-latest)	1m 53s
Nix (ubuntu-latest)	59s
Coq 8.16.1	11m 11s
Build (macos-latest, 4.14.x)	16m 43s
Coverage (4.14.x)	6h 0m 15s
Build (ubuntu-latest, 4.14.x)	15m 0s
MSVC 4.14.0 / windows_x86	9m 46s
Build (windows-latest, 4.14.x)	6m 24s
Documentation	38s
MSVC 4.14.0 / windows_x86_64	10m 52s
Format	2m 33s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	5m 34s
Build (5.1.x, ubuntu-latest, true)	5m 31s
Build (5.1.x, macos-latest, true)	4m 49s
Build monorepo benchmark docker image	1m 44s
Create local opam switch (macos-latest, 5)	8m 5s
Build (4.14.x, macos-13, true)	9m 34s
Create local opam switch (macos-latest, 4.14)	8m 32s
Build (4.13.x, ubuntu-latest, true)	4m 55s
Create local opam switch (ubuntu-latest, 5)	8m 23s
Build (4.08.x, ubuntu-latest, true)	5m 12s
Create local opam switch (ubuntu-latest, 4.14)	7m 59s
Build (4.04.x, ubuntu-latest, true, true)	7m 36s
Build (4.02.x, ubuntu-latest, true)	5m 45s
	8h 39m 53s

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