Update ADTypes requirement to 1.21.0

Updates the requirements on [ADTypes](https://github.com/SciML/ADTypes.jl) to permit the latest version.

Updates `ADTypes` to 1.21.0
- [Release notes](https://github.com/SciML/ADTypes.jl/releases)
- [Commits](v0.1.0...v1.21.0)

---
updated-dependencies:
- dependency-name: ADTypes
  dependency-version: 1.21.0
  dependency-type: direct:production
  dependency-group: all-julia-packages
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/Project.toml

@@ -3,4 +3,5 @@ ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 
 [compat]
-Documenter = "1.3"
+ADTypes = "1.21.0"
+Documenter = "1.3"