By Markus Wacket and Matthias Inverardi BERLIN (Reuters) – High coal prices, supply bottlenecks and aged plants are posing a challenge to Germany’s plan to increase electricity production from hard coal-fired power plants to compensate for declining gas deliveries, according to operators and industry experts. Earlier this month, Germany agreed to allow the reactivation of coal-fired power plants or an extension to their lifespans as it braces for falling Russian supplies of gas. Gas accounts for about 6% of the country’s electricity output. According to rough estimates, the move could compensa…