Switch from CLA to DCO.

See #892

CONTRIBUTING.adoc

@@ -18,10 +18,9 @@ Ideally, that would include a small sample project that reproduces the problem.
 If you think you have found a security vulnerability in Spring Vault please *DO NOT* disclose it publicly until we've had a chance to fix it.
 Please don't report security vulnerabilities using GitHub issues, instead head over to https://spring.io/security-policy and learn how to disclose them responsibly.
 
-== Sign the Contributor License Agreement
-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.
+== Developer Certificate of Origin (DCO)
+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].
 
 == Code Conventions and Housekeeping
 None of these is essential for a pull request, but they will all help.  They can also be

README.adoc

@@ -329,15 +329,13 @@ Follow the instructions for https://github.com/spring-io/spring-javaformat#eclip
 
 == Contributing
 
-Spring Vault is released under the non-restrictive Apache 2.0 license, and follows a very standard Github development process, using Github tracker for issues and merging pull requests into `main`.
+Spring Vault is released under the non-restrictive Apache 2.0 license, and follows a very standard GitHub development process, using GitHub tracker for issues and merging pull requests into `main`.
 If you want to contribute even something trivial please do not hesitate, but follow the guidelines below.
 
-=== Sign the Contributor License Agreement
+== Developer Certificate of Origin (DCO)
 
-Before we accept a non-trivial patch or pull request we will need you to sign the
-https://cla.pivotal.io/sign/spring[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].
 
 === Code of Conduct