Replace CLA with DCO in CONTRIBUTING guide

The Spring team has replaced the Contributor License Agreement with
the Developer Certificate of Origin. Update the contributing guide
to reflect this change and remove the defunct cla.pivotal.io link.

Fixes gh-3884

Signed-off-by: fru1tworld <fruitworld.planet@gmail.com>

Author: fru1tworld

.github/CONTRIBUTING.md

@@ -7,13 +7,12 @@ tracker for issues and merging pull requests into master. If you want
 to contribute even something trivial please do not hesitate, but
 follow the guidelines below.
 
-## Sign the Contributor License Agreement
-Before we accept a non-trivial patch or pull request we will need you to sign the
-[Contributor License Agreement](https://cla.pivotal.io/sign/spring).
-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.
+## Sign off commits with the Developer Certificate of Origin
+For all non-trivial patches or pull requests we need every commit to include a
+"Signed-off-by" trailer confirming that you agree to the
+[Developer Certificate of Origin](https://developercertificate.org/). See the
+[`git` documentation](https://git-scm.com/docs/git-commit#Documentation/git-commit.txt--s)
+if you are unfamiliar with how to do this.
 
 ## Code of Conduct
 This project adheres to the Contributor Covenant [Code of conduct](https://github.com/spring-cloud/spring-cloud-build/blob/main/docs/modules/ROOT/partials/code-of-conduct.adoc). By participating, you  are expected to uphold this code. Please report