Add a mutation to destroy LDAP admin role link records

What does this MR do and why?

Adds a mutation to destroy Authz::LdapAdminRoleLink records

References

Implements [BE] Add graphQL mutation to delete LDAP group ... (#524884 - closed).

Screenshots or screen recordings

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How to set up and validate locally

1. Enable relevant feature flags and seed some data:

   > Feature.enable(:custom_admin_roles)
   
   # Create an Authz::LdapAdminRoleLink. Take note of its global id
   > user_member_role = FactoryBot.create(:admin_member_role)
   > link = FactoryBot.create(:ldap_admin_role_link, member_role: user_member_role.member_role)
   > GitlabSchema.id_from_object(_)
   => #<GlobalID:0x0000000325e57310 @uri=#<URI::GID gid://gitlab/Authz::LdapAdminRoleLink/2>>

2. Log-in as admin and navigate to /-/graphql-explorer and run a query similar to the following (change the id):

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

3. Verify that the record has been deleted

   > Authz::LdapAdminRoleLink.find(2)
   ActiveRecord::RecordNotFound: Couldn't find Authz::LdapAdminRoleLink with 'id'=2

MR acceptance checklist

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