minor symfony#25035 Remove root v3 CHANGELOG and UPGRADE files (nicolas-grekas)

This PR was merged into the 4.0-dev branch.

Discussion
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Remove root v3 CHANGELOG and UPGRADE files

| Q             | A
| ------------- | ---
| Branch?       | 4.0
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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58cae8a Remove root v3 CHANGELOG and UPGRADE files

Author: nicolas-grekas