test: Drop testing on CentOS 8 Stream

C8S is going to be EOL in a month [1], and we are not going to do a RHEL
8 update at this point any more.

Keep testing on rhel-8-10 though, as we do want to keep the code working
for the beiboot scenario or backports.

[1] https://blog.centos.org/2023/04/end-dates-are-coming-for-centos-stream-8-and-centos-linux-7/

Makefile

@@ -3,7 +3,7 @@ PACKAGE_NAME := $(shell awk '/"name":/ {gsub(/[",]/, "", $$2); print $$2}' packa
 RPM_NAME := cockpit-$(PACKAGE_NAME)
 VERSION := $(shell T=$$(git describe 2>/dev/null) || T=1; echo $$T | tr '-' '.')
 ifeq ($(TEST_OS),)
-TEST_OS = centos-8-stream
+TEST_OS = centos-9-stream
 endif
 export TEST_OS
 TARFILE=$(RPM_NAME)-$(VERSION).tar.xz