Custom branch bug fix (#4215)

## What are you changing in this pull request and why?

Create release note for the custom branch bug fix

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
and [About
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
so my content adheres to these guidelines.
- [x] Needs review from product team

Adding new pages (delete if not applicable):
- [x] Add page to `website/sidebars.js`
- [x] Provide a unique filename for the new page

website/docs/docs/dbt-versions/release-notes/03-Oct-2023/custom-branch-fix-rn.md

@@ -0,0 +1,14 @@
+---
+title: "Fix: Default behavior for CI job runs without a custom branch"
+description: "October 2023: CI job runs now default to the main branch of the Git repository when a custom branch isn't set"
+sidebar_label: "Fix: Default behavior for CI job runs without a custom branch"
+tags: [Oct-2023, CI]
+date: 2023-10-06
+sidebar_position: 08
+---
+
+If you don't set a [custom branch](/docs/dbt-cloud-environments#custom-branch-behavior) for your dbt Cloud environment, it now defaults to the default branch of your Git repository (for example, `main`). Previously, [CI jobs](/docs/deploy/ci-jobs) would run for pull requests (PRs) that were opened against _any branch_ or updated with new commits if the **Custom Branch** option wasn't set. 
+
+## Azure DevOps 
+
+Your Git pull requests (PRs) might not trigger against your default branch if you're using Azure DevOps and the default branch isn't `main` or `master`. To resolve this, [set up a custom branch](/faqs/Environments/custom-branch-settings) with the branch you want to target.