zabbix: cherry-pick changes from 7.0.27 to 7.0.29 inclusive - #30217

Checking PR #30217: zabbix: cherry-pick changes from 7.0.27 to 7.0.29 inclusive (Makefile Audit)

Commit a993670 - zabbix: do not error in postinst when no ubusd:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 7548660 - zabbix: bump version to 7.0.28:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 97d15f0 - zabbix: preserve user config files across sysupgrade:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 10a5cdd - zabbix: reorganize files dir for clarity and to better follow the FHS:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 31e079e - zabbix: stop shipping sysctl.d conf in package:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 8c0060c - zabbix: add php8-mod-simplexml dependency to frontend:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 81400f7 - zabbix: use separate users for server and proxy:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 59eccff - zabbix: tweak patch agent and server patch configs:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 333d1aa - zabbix: tweak server initscript and bring agentd to parity:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 15311f3 - zabbix: bring proxy to parity with server and agentd:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.

Commit 17141a0 - zabbix: bump version to 7.0.29:

✅ Backport matches upstream commit verbatim. Skipping style and packaging validations.