[BE] Add graphQL mutation to delete LDAP group - custom admin role mapping

We need to add a mutation to delete an existing LDAP group - custom admin role mapping.

Only instance admin is allowed to do it.

Proposed mutation:

mutation {
  ldapAdminRoleLinkDestroy(input: {
    id: "gid://gitlab/Authz::LdapAdminRoleLink/2"
  }) {
    errors
    ldapAdminRoleLink {
      id
      memberRoleLink {
        id
      }
    }
  }
}