deprecate no longer required concat_alternatives

lhcb/Rec!3599 (merged) removes the last (unnecessary!) use of concat_alternatives from LHCb code -- so it is time to flag this code as deprecated, to be followed with removal at some point in the future.

GaudiFunctional/include/Gaudi/Functional/utilities.h

@@ -25,7 +25,7 @@ namespace Gaudi::Functional {
 
   template <typename... Strings>
   [[deprecated]] std::string concat_alternatives( const Strings&... s ) {
-    return concat_alternatives( std::initializer_list<std::string>{ s... } );
+    return boost::algorithm::join( std::initializer_list<std::string>{ s... }, ":" );
   }
 
   [[deprecated( "please use `updateHandleLocation` instead of `Gaudi::Functional::updateHandleLocation`" )]] inline void