minor symfony#62438 Remove outdated changelogs and upgrade guides in 8.0 (alexandre-daubois)

This PR was merged into the 8.0 branch.

Discussion
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 Remove outdated changelogs and upgrade guides in 8.0

| Q             | A
| ------------- | ---
| Branch?       | 8.0
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

Commits
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68ac0a4 Remove outdated changelogs and upgrade guides

Author: fabpot