fix: correct baseBranches description in onboarding PR

Closes #6527

lib/workers/repository/onboarding/pr/__snapshots__/base-branch.spec.ts.snap

@@ -5,3 +5,5 @@ exports[`workers/repository/onboarding/pr/base-branch getBaseBranchDesc() descri
 
 "
 `;
+
+exports[`workers/repository/onboarding/pr/base-branch getBaseBranchDesc() describes baseBranches 1`] = `"You have configured Renovate to use the following baseBranches: \`some-branch\`, \`some-other-branch\`."`;

lib/workers/repository/onboarding/pr/base-branch.spec.ts

@@ -14,7 +14,12 @@ describe('workers/repository/onboarding/pr/base-branch', () => {
       expect(res).toEqual('');
     });
     it('describes baseBranch', () => {
-      config.baseBranch = 'some-branch';
+      config.baseBranches = ['some-branch'];
+      const res = getBaseBranchDesc(config);
+      expect(res).toMatchSnapshot();
+    });
+    it('describes baseBranches', () => {
+      config.baseBranches = ['some-branch', 'some-other-branch'];
       const res = getBaseBranchDesc(config);
       expect(res).toMatchSnapshot();
     });

lib/workers/repository/onboarding/pr/base-branch.ts

@@ -2,7 +2,13 @@ import { RenovateConfig } from '../../../../config';
 
 export function getBaseBranchDesc(config: RenovateConfig): string {
   // Describe base branch only if it's configured
-  return config.baseBranch
-    ? `You have configured Renovate to use branch \`${config.baseBranch}\` as base branch.\n\n`
-    : '';
+  if (!config.baseBranches?.length) {
+    return '';
+  }
+  if (config.baseBranches.length > 1) {
+    return `You have configured Renovate to use the following baseBranches: ${config.baseBranches
+      .map((branch) => `\`${branch}\``)
+      .join(', ')}.`;
+  }
+  return `You have configured Renovate to use branch \`${config.baseBranches[0]}\` as base branch.\n\n`;
 }