Switch build host to Fedora 31

Change-Id: I6844c3b547b03c0faf4713b8a11a7915109250b7
diff --git a/.zuul.yaml b/.zuul.yaml
index a1c97b3..a053701 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -1,6 +1,6 @@
 - job:
     name: czechlight-clearfog
-    parent: run-ci-build-sh
+    parent: run-ci-build-sh-f31
     timeout: 3600
     vars:
       prepare_git_submodules: true
@@ -25,7 +25,7 @@
 
 - job:
     name: czechlight-beagleboneblack
-    parent: run-ci-build-sh
+    parent: run-ci-build-sh-f31
     timeout: 3600
     vars:
       prepare_git_submodules: true