CI: Build (via Buildroot) on Fedora 34

This bumps GCC to version 11, so there's that usual round of new
warnings-turned-errors that we have to fix.

Change-Id: I4aabd8038831d4e0260e630fbe2047be01a1f332
Depends-on: https://cesnet-gerrit-czechlight/c/ci/zuul-jobs-cesnet/+/4196/1
Depends-on: https://gerrit.cesnet.cz/c/ci/zuul-jobs-cesnet/+/4196/1
diff --git a/.zuul.yaml b/.zuul.yaml
index 51df62a..d8d2797 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -1,6 +1,6 @@
 - job:
     name: czechlight-clearfog
-    parent: run-ci-build-sh-f33
+    parent: run-ci-build-sh-f34
     timeout: 3600
     post-timeout: 600
     vars: