package/zabbix: fixup download URL

Fixes:
 http://autobuild.buildroot.org/results/9ff/9ffb4a9e3b2291498d0ccf3c9124bebc466860bc

Zabbix 6.2.x is EOL and the tarballs have now been moved under oldstable/.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
[Peter: extend commit message]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/zabbix/zabbix.mk

@@ -6,7 +6,7 @@
 
 ZABBIX_VERSION_MAJOR = 6.2
 ZABBIX_VERSION = $(ZABBIX_VERSION_MAJOR).7
-ZABBIX_SITE = https://cdn.zabbix.com/zabbix/sources/stable/$(ZABBIX_VERSION_MAJOR)
+ZABBIX_SITE = https://cdn.zabbix.com/zabbix/sources/oldstable/$(ZABBIX_VERSION_MAJOR)
 ZABBIX_SELINUX_MODULES = zabbix
 ZABBIX_LICENSE = GPL-2.0+
 ZABBIX_LICENSE_FILES = README COPYING