[Security] Add SymfonyCasts as a backer of version 6.3

Author: nicolas-grekas

README.md

@@ -7,7 +7,7 @@ The Security CSRF (cross-site request forgery) component provides a class
 Sponsor
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-The Security component for Symfony 6.2 is [backed][1] by [SymfonyCasts][2].
+The Security component for Symfony 6.3 is [backed][1] by [SymfonyCasts][2].
 
 Learn Symfony faster by watching real projects being built and actively coding
 along with them. SymfonyCasts bridges that learning gap, bringing you video