Switch to Fedora 31 builds

We still have GCC 8 in the ARM targets, so let's make sure we're using
one target with a similar compiler during our project builds.

Change-Id: I58ab5c2e61dd4effff027d9d02ead72a88a3bd2c
Depends-on: https://gerrit.cesnet.cz/c/CzechLight/dependencies/+/2481
Depends-on: https://gerrit.cesnet.cz/c/ci/zuul-jobs-cesnet/+/2481
diff --git a/.zuul.yaml b/.zuul.yaml
index 792b363..1ddd416 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -2,17 +2,19 @@
     check:
       fail-fast: true
       jobs:
-        - f29-gcc-cover:
+        - f31-gcc-cover:
+            requires: CzechLight-deps-f31-gcc
+        - f31-gcc-asan-ubsan:
+            requires: CzechLight-deps-f31-gcc-asan-ubsan
+        - f31-clang-asan:
+            requires: CzechLight-deps-f31-clang-asan
+        - f31-clang-tsan:
+            requires: CzechLight-deps-f31-clang-tsan
+        - f29-gcc:
             requires: CzechLight-deps-f29-gcc
-        - f29-gcc-asan-ubsan:
-            requires: CzechLight-deps-f29-gcc-asan-ubsan
-        - f29-clang-asan:
-            requires: CzechLight-deps-f29-clang-asan
-        - f29-clang-tsan:
-            requires: CzechLight-deps-f29-clang-tsan
-        - cpp-coverage-diff:
+        - f31-cpp-coverage-diff:
             voting: false
         - clang-format:
             voting: false
-        - f29-gcc-cover-previous:
-            requires: CzechLight-deps-f29-gcc
+        - f31-gcc-cover-previous:
+            requires: CzechLight-deps-f31-gcc