Bump version to 1.21.0

Preparing release for changes since v1.20.0:
- Fix docstring typos and add missing backends to Auto function
- Migrate from CompatHelper to Dependabot
- Update dependabot.yml for Julia ecosystem support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ChrisRackauckas

Project.toml

@@ -1,7 +1,7 @@
 name = "ADTypes"
 uuid = "47edcb42-4c32-4615-8424-f2b9edc5f35b"
 authors = ["Vaibhav Dixit <vaibhavyashdixit@gmail.com>, Guillaume Dalle and contributors"]
-version = "1.20.0"
+version = "1.21.0"
 
 [weakdeps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"