feat(dependabot): add target-branch configuration (#3537)

Adding missing target-branch option to the dependabot configuration.

---
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

docs/api/github.md

@@ -4838,6 +4838,7 @@ const dependabotOptions: github.DependabotOptions = { ... }
 | <code><a href="#projen.github.DependabotOptions.property.registries">registries</a></code> | <code>{[ key: string ]: <a href="#projen.github.DependabotRegistry">DependabotRegistry</a>}</code> | Map of package registries to use. |
 | <code><a href="#projen.github.DependabotOptions.property.reviewers">reviewers</a></code> | <code>string[]</code> | Specify individual reviewers or teams of reviewers for all pull requests raised for a package manager. |
 | <code><a href="#projen.github.DependabotOptions.property.scheduleInterval">scheduleInterval</a></code> | <code><a href="#projen.github.DependabotScheduleInterval">DependabotScheduleInterval</a></code> | How often to check for new versions and raise pull requests. |
+| <code><a href="#projen.github.DependabotOptions.property.targetBranch">targetBranch</a></code> | <code>string</code> | https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#target-branch You can configure the target branch for raising pull requests for version updates against. |
 | <code><a href="#projen.github.DependabotOptions.property.versioningStrategy">versioningStrategy</a></code> | <code><a href="#projen.github.VersioningStrategy">VersioningStrategy</a></code> | The strategy to use when edits manifest and lock files. |
 
 ---
@@ -4986,6 +4987,18 @@ How often to check for new versions and raise pull requests.
 
 ---
 
+##### `targetBranch`<sup>Optional</sup> <a name="targetBranch" id="projen.github.DependabotOptions.property.targetBranch"></a>
+
+```typescript
+public readonly targetBranch: string;
+```
+
+- *Type:* string
+
+https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#target-branch You can configure the target branch for raising pull requests for version updates against.
+
+---
+
 ##### `versioningStrategy`<sup>Optional</sup> <a name="versioningStrategy" id="projen.github.DependabotOptions.property.versioningStrategy"></a>
 
 ```typescript

src/github/dependabot.ts

@@ -91,6 +91,12 @@ export interface DependabotOptions {
    * @default []
    */
   readonly groups?: { [name: string]: DependabotGroup };
+
+  /**
+   * https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#target-branch
+   * You can configure the target branch for raising pull requests for version updates against
+   */
+  readonly targetBranch?: string;
 }
 
 /**
@@ -390,6 +396,7 @@ export class Dependabot extends Component {
             options.openPullRequestsLimit !== undefined
               ? options.openPullRequestsLimit
               : undefined,
+          "target-branch": options.targetBranch,
         },
       ],
     };

test/github/dependabot.test.ts

@@ -280,6 +280,36 @@ describe("dependabot", () => {
       expect(dependabot).toContain("dependency-name: projen");
     });
   });
+
+  describe("target branch", () => {
+    test("empty target branch", () => {
+      const project = createProject();
+
+      new Dependabot(project.github!, {});
+
+      const snapshot = synthSnapshot(project);
+      const dependabot = snapshot[".github/dependabot.yml"];
+      expect(dependabot).toBeDefined();
+      expect(dependabot).toMatchSnapshot();
+      expect(dependabot).not.toContain("target-branch4");
+    });
+
+    test("develop as the target branch", () => {
+      const project = createProject();
+
+      const targetBranch = "develop";
+
+      new Dependabot(project.github!, {
+        targetBranch: targetBranch,
+      });
+
+      const snapshot = synthSnapshot(project);
+      const dependabot = snapshot[".github/dependabot.yml"];
+      expect(dependabot).toBeDefined();
+      expect(dependabot).toMatchSnapshot();
+      expect(dependabot).toContain(`target-branch: ${targetBranch}`);
+    });
+  });
 });
 
 type ProjectOptions = Omit<