Update gas price calculation in CommunityPool contract

[yavrsky]

During a recent investigation, it was found that block.basefee in the call returns 0, so it was decided to return tx.gasprice back and set it ourselves during the call to getRecommendedRechargeAmount. Also, in the _balanceIsSufficient method, tx.gasprice was replaced with block.basefee, which avoids manipulation of the gasprice during the call to rechargeUserWallet.