Update documents about greenkeeper in favor of dependabot (#4708)

docs/developer/dependency-management.md

@@ -14,8 +14,8 @@
 
 - Use [npm](https://www.npmjs.com/) as package manager.
 - Run `npm ci` after checking out the repository to install dependencies.
-- [Greenkeeper](https://greenkeeper.io/) updates packages by creating pull requests for
-the new releases of used packages. Its pull requests are marked with `greenkeeper` label.
+- [Dependabot Preview](https://github.com/marketplace/dependabot-preview/) updates packages by creating pull requests for
+the new releases of used packages. Its pull requests are marked with `area/dependency` label.
 - Update `package-lock.json` before sending any changes.
 
 ----

docs/user/labels.md

@@ -53,7 +53,7 @@ Labels used to determine if creator of pull request has signed the CNCF CLA. It
 
 Other labels used for issues and pull requests.
 
-* `greenkeeper` - pull requests created by [greenkeeper.io](https://greenkeeper.io/) bot. This label is assigned automatically.
+* `area/dependency` - pull requests created by [Dependabot Preview](https://github.com/marketplace/dependabot-preview/) bot. This label is assigned automatically.
 * `help wanted` - rather low priority issues, where any contributions are more than welcomed.
 * `good first issue` - issues good for contributor onboarding.
 * `missing details` - issues with missing details, that cannot be reproduced.