Switch from CLA to DCO

Closes gh-43734

Author: philwebb

.github/dco.yml

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

CONTRIBUTING.adoc

@@ -29,11 +29,10 @@ Please don't report security vulnerabilities using GitHub issues, instead head o
 
 
 
-== Sign the Contributor License Agreement
+== Include a Signed Off By Trailer
 
-Before we accept a non-trivial patch or pull request we will need you to https://cla.pivotal.io/sign/spring[sign the Contributor License Agreement].
-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.
-Active contributors might be asked to join the core team, and given the ability to merge pull requests.
+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.
+For additional details, please refer to the blog post https://spring.io/blog/2025/01/06/hello-dco-goodbye-cla-simplifying-contributions-to-spring[Hello DCO, Goodbye CLA: Simplifying Contributions to Spring].