Update GitLab CI to Fedora 31

Since Fedora 31 is the current Fedora release, replace Fedora 30 GitLab
CI jobs with their up-to-date counterparts.

(cherry picked from commit b36f5496237f0dbb84d7541140e87d7da475cd36)

.gitlab-ci.yml

@@ -106,8 +106,8 @@ stages:
 
 # Fedora
 
-.fedora-30-amd64: &fedora_30_amd64_image
-  image: "$CI_REGISTRY_IMAGE:fedora-30-amd64"
+.fedora-31-amd64: &fedora_31_amd64_image
+  image: "$CI_REGISTRY_IMAGE:fedora-31-amd64"
   <<: *linux_amd64
 
 # Ubuntu
@@ -715,29 +715,29 @@ unit:gcc:sid:i386:
     - gcc:sid:i386
   needs: ["gcc:sid:i386"]
 
-# Jobs for regular GCC builds on Fedora 30 (amd64)
+# Jobs for regular GCC builds on Fedora 31 (amd64)
 
-gcc:fedora30:amd64:
+gcc:fedora31:amd64:
   variables:
     CC: gcc
     CFLAGS: "-Wall -Wextra -O2 -g"
     EXTRA_CONFIGURE: "--with-libidn2"
-  <<: *fedora_30_amd64_image
+  <<: *fedora_31_amd64_image
   <<: *build_job
 
-system:gcc:fedora30:amd64:
-  <<: *fedora_30_amd64_image
+system:gcc:fedora31:amd64:
+  <<: *fedora_31_amd64_image
   <<: *system_test_job
   dependencies:
-    - gcc:fedora30:amd64
-  needs: ["gcc:fedora30:amd64"]
+    - gcc:fedora31:amd64
+  needs: ["gcc:fedora31:amd64"]
 
-unit:gcc:fedora30:amd64:
-  <<: *fedora_30_amd64_image
+unit:gcc:fedora31:amd64:
+  <<: *fedora_31_amd64_image
   <<: *unit_test_job
   dependencies:
-    - gcc:fedora30:amd64
-  needs: ["gcc:fedora30:amd64"]
+    - gcc:fedora31:amd64
+  needs: ["gcc:fedora31:amd64"]
 
 # Jobs for regular GCC builds on Ubuntu 16.04 Xenial Xerus (amd64)
 

PLATFORMS

@@ -20,7 +20,7 @@ following systems:
 
   * Debian 8, 9, 10
   * Ubuntu 16.04, 18.04
-  * Fedora 28, 29
+  * Fedora 31
   * Red Hat Enterprise Linux / CentOS 6, 7
   * FreeBSD 11.x
   * OpenBSD 6.2, 6.3

PLATFORMS.md

@@ -28,7 +28,7 @@ following systems:
 
 * Debian 8, 9, 10
 * Ubuntu 16.04, 18.04
-* Fedora 28, 29
+* Fedora 31
 * Red Hat Enterprise Linux / CentOS 6, 7
 * FreeBSD 11.x
 * OpenBSD 6.2, 6.3