ci: update keyserver URL

The `sks-keyservers.net` keyserver pool is no longer provided.
Switch to `keyserver.ubuntu.com` instead.

.gitlab/ci/docker/debian10/install_rvm.sh

@@ -8,7 +8,7 @@ apt-get install -y \
     gnupg2 \
     procps
 
-gpg2 --keyserver hkp://pool.sks-keyservers.net \
+gpg2 --keyserver hkps://keyserver.ubuntu.com \
      --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 \
                  7D2BAF1CF37B13E2069D6956105BD0E739499BDB
 

.gitlab/ci/docker/fedora34/install_rvm.sh

@@ -2,7 +2,7 @@
 
 set -e
 
-gpg2 --keyserver hkp://pool.sks-keyservers.net \
+gpg2 --keyserver hkps://keyserver.ubuntu.com \
      --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 \
                  7D2BAF1CF37B13E2069D6956105BD0E739499BDB