Update changelog to indicate dropping support of GHC 7.4

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Cabal/changelog

 -*-change-log-*-
 
 1.25.x.x (current development version)
-	* Dropped support for versions of GHC earlier than 6.12 (#3111).
+	* Dropped support for versions of GHC earlier than 7.6 (#3833).
 	* Convenience/internal libraries are now supported (#269).
 	  An internal library is declared using the stanza "library
 	  'libname'".  Packages which use internal libraries can