minor symfony#24081 [Dotenv] Add a BC break note (voronkovich)

This PR was squashed before being merged into the 3.3 branch (closes symfony#24081).

Discussion
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[Dotenv] Add a BC break note

| Q             | A
| ------------- | ---
| Branch?       | 3.3
| Bug fix?      | yes
| New feature?  |no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#24078
| License       | MIT

Commits
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66621cc [Dotenv] Add a BC break note

Author: nicolas-grekas

src/Symfony/Component/Dotenv/CHANGELOG.md

@@ -4,4 +4,5 @@ CHANGELOG
 3.3.0
 -----
 
+ * [BC BREAK] Since v3.3.7, the latest Dotenv files override the previous ones. Real env vars are not affected and are not overridden.
  * added the component