[snap] move to core20

Don't install apt packged related to ocl

snap/snapcraft.yaml

@@ -1,6 +1,6 @@
 name: clpeak
-base: core18
-version: '1.0'
+base: core20
+version: "1.0"
 version-script: "echo `grep 'VERSION_MAJOR ' cmake/common.cmake | awk -F'[() ]' '{print $3}'`.`grep 'VERSION_MINOR ' cmake/common.cmake | awk -F'[() ]' '{print $3}'`.`grep 'VERSION_PATCH ' cmake/common.cmake | awk -F'[() ]' '{print $3}'`"
 summary: Benchmarking tool to measure peak capabilities of opencl devices
 description: |
@@ -20,8 +20,3 @@ parts:
     plugin: cmake
     build-packages:
       - build-essential
-      - opencl-clhpp-headers
-      - ocl-icd-opencl-dev
-    stage-packages:
-      - ocl-icd-libopencl1
-