Add deprecation warning to LegacyPdoSessionHandler

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

LegacyPdoSessionHandler has been deprecated in favor of PdoSessionHandler. Trigger an E_USER_DEPRECATED error when the deprecated class is used.

Addresses @fabpot's comment on [#13068](https://github.com/symfony/symfony/pull/13068#issuecomment-67977358).