Update Spark changelog for 2.7.0 release
Update Spark changelog for 2.7.0 release

BittyByte/Spark,speedy01/Spark,BittyByte/Spark,guusdk/Spark,guusdk/Spark,wrooot/Spark,vipinraj/Spark,guusdk/Spark,speedy01/Spark,speedy01/Spark,wrooot/Spark,vipinraj/Spark,wrooot/Spark,igniterealtime/Spark,vipinraj/Spark,wrooot/Spark,speedy01/Spark,BittyByte/Spark,guusdk/Spark,igniterealtime/Spark,vipinraj/Spark,igniterealtime/Spark,BittyByte/Spark,igniterealtime/Spark