This PR was squashed before being merged into the 4.1 branch (closes #27472).

Discussion
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[DI] Ignore missing tree root nodes on validate

| Q             | A
| ------------- | ---
| Branch?       | 4.1
| Bug fix?      | yes
| New feature?  | technically yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #27450
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

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Commits
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b3cdfc6 [DI] Ignore missing tree root nodes on validate

…abbuh)

This PR was merged into the 4.2-dev branch.

Discussion
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[Config] deprecate tree builders without root nodes

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

While reviewing #27472 I wondered if we really need support config trees without a root node. If we did not support it, users wouldn't create pseudo configuration classes when they were actually not needed.

Commits
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c2ce153 deprecate tree builders without root nodes