minor #14736 [String] Restore version-added (jderusse)

This PR was merged into the 5.1 branch.

Discussion
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[String] Restore version-added

This restore the `version-added` removed too early.

spotted here #14735 (comment)

Commits
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275b6e9 Restore version-added in string

Author: wouterj

components/string.rst

@@ -8,6 +8,10 @@ The String Component
     The String component provides a single object-oriented API to work with
     three "unit systems" of strings: bytes, code points and grapheme clusters.
 
+.. versionadded:: 5.0
+
+    The String component was introduced in Symfony 5.0.
+
 Installation
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