Update release-playbook.md to update the MODULE.bazel (#2161)

Adds a new step to update the MODULE.bazel.

docs/release-playbook.md

@@ -50,6 +50,14 @@ These steps only have to be performed once, ever.
         *   A first release candidate should immediately be created after the release branch is created. See [create a release candidate](#create-a-release-candidate) below.
 1.  After creating the branch, edit the CODEOWNERS file on that branch, replace the entire contents of the file with the line `* @your-github-username` and submit it directly.
     *   This makes sure that all PRs sent against that branch have you as a reviewer.
+1.  Update the MODULE.bazel file in the new branch. Change the version in the module to the version of the new release branch. For example, if the new release branch is `release-8.1.0`, then change the version to `8.1.0` like below.
+    * ```
+        module(
+            name = "bazel",
+            version = "8.1.0",
+            repo_name = "io_bazel",
+        )
+      ```
 1.  Update the branch name in the scheduled build for release branches by editing the "build branch" field [here](https://buildkite.com/bazel/bazel-at-head-plus-downstream/settings/schedules/b63d6589-2658-4850-a9b9-b588b9ea5c99/edit). Set it to `release-X.Y.Z` so that downstream tests run against this branch.
 1.  Ping [@meteorcloudy](https://github.com/meteorcloudy) to configure a [GitHub merge queue](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#about-merge-queues) for this new branch.
 1.  (For minor release only) Send an email to [email protected] announcing the next release.