Replace EOL centos CI with rockylinux

centos7 has reached EOL on 2024-06-30, centos8 on 2021-10-31. In
practical terms this means that their package repositories are offline
for the foreseeable future and the CI tasks making use of them might
never run again.

This patch replaces the existing CI jobs on centos7/centos8 with jobs on
rockylinux8/9. Since rockylinux is an open source variant similar to
RHEL this should provide testing with similar spirit. In contrast to
centos, rockylinux does provide very granular tagging of releases which
if used would help avoid breaking on e.g., subtle package changes in
their package repositories. I however did not use very precise tags in
the patch and instead went with broad "trains" for the 8 and 9 series;
I hope this minimizes maintenance overhead (e.g., bumping to new
releases) while still giving a good testing signal.

.cirrus.yml

@@ -11,25 +11,24 @@ freebsd_task:
   test_script: |
     sudo -u testuser .ci/unix-test.sh
 
-centos7_task:
+rockylinux8_task:
   container:
-    image: centos:7
+    image: docker.io/rockylinux:8
   install_script: |
-    yum install -y centos-release-scl
-    yum install -y devtoolset-9
-    curl -L https://github.com/Kitware/CMake/releases/download/v3.16.4/cmake-3.16.4-Linux-x86_64.tar.gz | tar xzvf - -C /usr/local --strip-components 1
+    dnf group install -y "Development Tools"
+    dnf install cmake -y
   build_script: |
-    source /opt/rh/devtoolset-9/enable && PATH=$PATH:/usr/local/bin .ci/unix-build.sh
+    .ci/unix-build.sh
   test_script: |
-    PATH=$PATH:/usr/local/bin .ci/unix-test.sh
+    .ci/unix-test.sh
 
-centos8_task:
+rockylinux9_task:
   container:
-    image: quay.io/centos/centos:stream8
+    image: docker.io/rockylinux:9
   install_script: |
-    yum group install -y "Development Tools"
-    curl -L https://github.com/Kitware/CMake/releases/download/v3.16.4/cmake-3.16.4-Linux-x86_64.tar.gz | tar xzvf - -C /usr/local --strip-components 1
+    dnf group install -y "Development Tools"
+    dnf install cmake -y
   build_script: |
-    PATH=$PATH:/usr/local/bin .ci/unix-build.sh
+    .ci/unix-build.sh
   test_script: |
-    PATH=$PATH:/usr/local/bin .ci/unix-test.sh
+    .ci/unix-test.sh