ci: update to Fedora 33 for Linux builds

.gitlab/ci/docker/fedora31/Dockerfile → .gitlab/ci/docker/fedora33/Dockerfile

-FROM fedora:31 as rvm-build
+FROM fedora:33 as rvm-build
 MAINTAINER Ben Boeckel <ben.boeckel@kitware.com>
 
 COPY install_rvm.sh /root/install_rvm.sh
 RUN sh /root/install_rvm.sh
 
-FROM fedora:31
+FROM fedora:33
 MAINTAINER Ben Boeckel <ben.boeckel@kitware.com>
 
 COPY install_deps.sh /root/install_deps.sh

.gitlab/ci/docker/fedora31/install_rvm.sh → .gitlab/ci/docker/fedora33/install_rvm.sh

@@ -13,6 +13,9 @@ dnf install --setopt=install_weak_deps=False -y \
 
 curl -sSL https://get.rvm.io | bash -s stable
 
+# This is intentionally an older version.
+# If updating, the associated `env_fedora*_makefiles.cmake` file needs updated
+# as well.
 /usr/local/rvm/bin/rvm install ruby-2.7.0
 
 tar -C /usr/local -cf /root/rvm.tar rvm