Install Menu Block for the welcome dashboard's Add content block

Problem/Motivation

The welcome dashboard (config/dashboard.dashboard.welcome.yml, from #3629449) places the "Add content" block as the menu_block:content plugin and lists menu_block in its module dependencies. Varbase Admin Base does not require drupal/menu_block in composer.json and does not install it in recipe.yml, so it only works when Varbase Content Base is applied first.

A clean drush site:install varbase from the refreshed Varbase Project 11.0.11 lock warned that the menu_block:content block plugin was not found while the dashboard config was installed. The Vardot site templates apply the recipes in an order that hides it.

Steps to reproduce

1. Build Varbase 11 from the Varbase Project lock.
2. Install it with drush site:install varbase and watch for the menu_block:content plugin not found warning.
3. Open /admin/dashboard and check the "Add content" block.

Proposed resolution

• Require "drupal/menu_block": "~1" in composer.json, the constraint Varbase Content Base uses.
• Add menu_block to the install: list in recipe.yml, which runs before the dashboard config is imported.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Update Release Notes
• ✅ Release Varbase Admin Base 1.0.6

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A