How do I create a configuration menu with the selected section name?
I tested it with your code and you just have to change two little things in your custom_menu.adminmenu_section. The rest is ok. See below:

module.links.menu.yml

custom_menu.adminmenu_section:
  title: 'Configuration' &lt;= change this to your section name
  description: 'Custom Menus Configuration section'    
  parent: system.admin_config  &lt;= change this to system.admin_config
  route_name: custom_menu.adminmenu_section