Update changelog

CHANGELOG.md

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 ### Added
 
+- Introduce reducable demand. It should be listed within sinks, and provided an efficiency (number between 0 and 1). This efficiency correspond to the percent of the demand which must be provided (critical demand). The oemof-solph sinks which models the  non-critical part of the demand has very small variable_costs such that it should not influence the costs calculations but should be fulfilled rather than dumping energy into excess sinks. Developed for the server version. (#969)
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 ### Changed
 
+- Add costs to excess sinks of busses. If the dictionary containing the information about the bus contains a key "price", its value will be applied to the variable costs of the sink (unit of the price is currency/energy unit, default currency/kWh). Developed for the server version. (#969)
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 ### Fixed
 
 ## [1.1.0] - 2024-04-27