update RT kernel to 5.15.70 RT 50

compile.bash

 #!/bin/bash
 
 # get current kernel and realtime patch version
-VERSION=5.14
+VERSION=5.15
 FILE=`curl -s https://mirrors.edge.kernel.org/pub/linux/kernel/projects/rt/${VERSION}/`
-KERNEL=2
-RTPATCH=21
+KERNEL=70
+RTPATCH=50
 echo "using kernel $VERSION.$KERNEL with realtime patch $RTPATCH"
 
 # get kernel file names and location