minor #15129 Update bc.rst (94noni)

This PR was merged into the 4.4 branch.

Discussion
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Update bc.rst

To map number version from https://symfony.com/doc/current/contributing/community/releases.html#backward-compatibility

Also i think mentioning those internal deprecation messages can be useful here in this doc section.

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Commits
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6e3a415 Update bc.rst

Author: javiereguiluz

contributing/code/bc.rst

@@ -4,10 +4,13 @@ Our Backward Compatibility Promise
 Ensuring smooth upgrades of your projects is our first priority. That's why
 we promise you backward compatibility (BC) for all minor Symfony releases.
 You probably recognize this strategy as `Semantic Versioning`_. In short,
-Semantic Versioning means that only major releases (such as 2.0, 3.0 etc.) are
-allowed to break backward compatibility. Minor releases (such as 2.5, 2.6 etc.)
+Semantic Versioning means that only major releases (such as 5.0, 6.0 etc.) are
+allowed to break backward compatibility. Minor releases (such as 5.1, 5.2 etc.)
 may introduce new features, but must do so without breaking the existing API of
-that release branch (2.x in the previous example).
+that release branch (5.x in the previous example).
+
+We also provide deprecation message triggered in the code base to help you with
+the migration process across major release.
 
 .. caution::