feature symfony#24024 [config] Add ability to deprecate a node (sanpii)

This PR was merged into the 3.4 branch.

Discussion
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[config] Add ability to deprecate a node

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This is symfony#22382 on 3.4

Commits
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a00a4ff [config] Add ability to deprecate a node

Author: nicolas-grekas