Switch from CLA to DCO

Author: wilkinsona

.github/dco.yml

@@ -0,0 +1,2 @@
+require:
+  members: false

CONTRIBUTING.md

@@ -8,11 +8,10 @@ If you would like to contribute something, or simply want to work with the code,
 This project adheres to the Contributor Covenant [code of conduct][1].
 By participating, you are expected to uphold this code. Please report unacceptable behavior to spring-code-of-conduct@pivotal.io.
 
-## Sign the contributor license agreement
-
-Before we accept a non-trivial patch or pull request we will need you to sign the [contributor's license agreement (CLA)][2].
-Signing the contributor's agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do.
+## Include a Signed-off-by trailer
 
+All commits must include a _Signed-off-by_ trailer at the end of each commit message to indicate that the contributor agrees to the [Developer Certificate of Origin (DCO)][2].
+For additional details, please refer to the ["Hello DCO, Goodbye CLA: Simplifying Contributions to Spring"][3] blog post.
 
 ## Code conventions and housekeeping
 
@@ -24,7 +23,7 @@ None of these is essential for a pull request, but they will all help
 - Add some Javadocs
 - Add unit tests that cover any new or modified functionality
 - Whenever possible, please rebase your branch against the current main (or other target branch in the main project).
-- When writing a commit message please follow [these conventions][3].
+- When writing a commit message please follow [these conventions][4].
   Also, if you are fixing an existing issue please add `Fixes gh-nnn` at the end of the commit message (where nnn is the issue number).
 
 ## Working with the code
@@ -38,5 +37,6 @@ $ ./gradlew build
 ```
 
 [1]: CODE_OF_CONDUCT.md
-[2]: https://cla.pivotal.io/sign/spring
-[3]: https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+[2]: https://en.wikipedia.org/wiki/Developer_Certificate_of_Origin
+[3]: https://spring.io/blog/2025/01/06/hello-dco-goodbye-cla-simplifying-contributions-to-spring
+[4]: https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html