zabbix_agentd: Update to 6.0.21 (LTS)

zabbix_agentd: Update to 6.0.21 (LTS)

- Update from version 6.0.19 to 6.0.21
- Update of rootfile not required

Bugs fixed:
- ZBX-23097:
Fixed use of uninitialised value when verifying subject and issuer with
TLS
- ZBX-22871:
Fixed regular expression crash with invalid utf-8 sequences when pcre2
is used
- ZBX-23221:
Fixed memory leaks when using certificate-based encryption
- ZBX-18168:
Added regexp runtime error logging for log*[] items

Full changelogs since 6.0.19:
- https://www.zabbix.com/rn/rn6.0.20
- https://www.zabbix.com/rn/rn6.0.21

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>