Update ADTypes requirement to 1.21.0 in /docs

Updates the requirements on [ADTypes](https://github.com/SciML/ADTypes.jl) to permit the latest version.
- [Release notes](https://github.com/SciML/ADTypes.jl/releases)
- [Commits](SciML/ADTypes.jl@v0.1.0...v1.21.0)

---
updated-dependencies:
- dependency-name: ADTypes
  dependency-version: 1.21.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

docs/Project.toml

@@ -15,5 +15,6 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 
 [compat]
+ADTypes = "1.21.0"
 Documenter = "1.3"
 Literate = "2.21.0"