bug #42354 [Ldap][Security] Make LdapAuthenticator an EntryPoint (dcp-dev, chalasr)

This PR was merged into the 5.3 branch.

Discussion
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[Ldap][Security] Make LdapAuthenticator an EntryPoint

| Q             | A
| ------------- | ---
| Branch?       | 5.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #42346
| License       | MIT
| Doc PR        | N/A

I added `@chalasr`'s recommandations given in https://github.com/symfony/symfony/issues/42346#issuecomment-891188282

Commits
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4daad9e7 Fix decorating non-entrypoint authenticators
0b0c15c0 [Ldap] Make LdapAuthenticator an EntryPoint