minor symfony#52794 Remove 6.x changelogs (derrabus)

This PR was merged into the 7.0 branch.

Discussion
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Remove 6.x changelogs

| Q             | A
| ------------- | ---
| Branch?       | 7.0
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | N/A
| License       | MIT

We usually don't keep changelogs of the previous major version around.

Commits
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012116b Remove 6.x changelogs

Author: fabpot