[ci] Move from OpenSUSE Leap 15 to Leap 16

OpenSUSE Leap 15 is end-of-life, so we move the special `march=native`
build on OpenSUSE to Leap 16.

Author: guitargeek

.github/workflows/root-ci-config/buildconfig/opensuse15-march_native.txt

@@ -438,8 +438,8 @@ jobs:
           #   property: gpu
           #   extra-runs-on: gpu
           # openSUSE Leap
-          - image: opensuse15
-            platform_config: opensuse15-march_native
+          - image: opensuse16
+            platform_config: opensuse16-march_native
             is_special: true
             property: march_native
 
@@ -662,4 +662,4 @@ jobs:
       KRB5REALM: ${{ secrets.KRB5REALM }}
       KRB5USER: ${{ secrets.KRB5USER }}
     with:
-      branch: ${{ github.head_ref || github.ref_name }}
+      branch: ${{ github.head_ref || github.ref_name }}