[Security][Acl] Fix #5787 : Add MutableAclProvider::updateUserSecurityIdentity
feature #8650 [Security][Acl] Add MutableAclProvider::updateUserSecurityIdentity (lemoinem)

This PR was merged into the 2.5-dev branch.

Discussion
----------

[Security][Acl] Add MutableAclProvider::updateUserSecurityIdentity

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5787
| License       | MIT
| Doc PR        | symfony/symfony-docs#3319

This provides a very simple function to enable the update of a User's username while keeping its associated ACEs (by updating its corresponding UserSecurityIdentity)

Developers can add a listener on the preUpdate of a user and remove all the related ACLs:

```php
if ($args->hasChangedField('username')) {
    $aclProvider = $this->container->get('security.acl.provider');

    $oldUsername = $args->getOldValue ('username');
    $user        = $args->getEntity();

    $aclProvider->updateUserSecurityIdentity(UserSecurityIdentity::fromAccount($user) , $oldUsername);
}
```
Among the problems of not updating the UserSecurityIdentity:

- Inconsistent database, referring to a non-existent user.
- The user loses all its associated permissions
- If another user is created with the old username, it will inherit all the first user’s ACEs

This PR intends to fix Issue #5787 and is similar to and inspired from PR #8305.
This will also be heavily impacted by the outcome of #8848

Commits
-------

da53d92 [Security][Acl] Fix #5787 : Add MutableAclProvider::updateUserSecurityIdentity