This PR was merged into the 4.3-dev branch.

Discussion
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Drop more usages of Serializable

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

When the serialized payload of some class is used for ephemeral needs, my proposal here is to drop implementing `Serializable` and provide a deprecation layer based on `__sleep` to still call the `serialize`/`unserialize` methods. This means that at the behavior level, we provide BC, and at the payload level, we don't.

That leaves us with only two "not ephemeral" use cases:
- `Route` and `CompilerRoute` instances are serialized by Drupal. We'd better not break such existing payloads without being sure it's ok for Drupal (ping @alexpott). My proposal in this PR is to schedule the BC break for Symfony 5, and ask Drupal (and others) to add a check in their unserialization logic so that they are able to recompile the cached list of routes if unserialize fails due to this change. They could alternatively implement `Serializable` on their own, without calling `parent::(un)serialize()`.
- `TokenInterface` from `Security` - for this one, I have no idea yet, except either plan for breaking BC at the payload level (which would mean invalidating all sessions when moving to SF5) - or create a new interface if we think breaking sessions is not reasonable. For now, I've kept using `Serializable`, which could be also fine keeping if we mark the corresponding method `final` to force

WDYT?

Commits
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05d6475 Drop more usages of Serializable

…(Tobion)

This PR was merged into the 4.3 branch.

Discussion
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[Routing] revert deprecation of Serializable in routing

| Q             | A
| ------------- | ---
| Branch?       | 4.3
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets |
| License       | MIT
| Doc PR        |

we still need to implement Serializable as long as we support PHP < 7.4. otherwise serialized data in php 7.2 would not work anymore when people upgrade to php 7.4. see discussion in #31792 (comment)
partly reverts #30286

Commits
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d32a295 [Routing] revert deprecation of Serializable in routing

This PR was merged into the 5.0-dev branch.

Discussion
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[Routing] remove deprecations

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | yes     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets |
| License       | MIT
| Doc PR        |

Ref. #30249, #30286, #24894

Commits
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5beb5f8 [Routing] remove deprecations