Update the docs about Journald support. (#1272)

This PR updates the docs stating in which Container variantes the
Journald dependencies are shipped, allowing Elastic-Agent to run
inputs/integrations that read from journald like: system, journald and
iptables.

The changes on this PR need to be released together with
elastic/elastic-agent#7995 and its backports.

The docs can be previewed at:
https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1272/reference/fleet/install-elastic-agents#flavors-for-container-package-installs

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: David Kilfoyle <[email protected]>
Co-authored-by: Brandon Morelli <[email protected]>
Co-authored-by: Brandon Morelli <[email protected]>

Author: 4 people

reference/fleet/install-elastic-agents.md

@@ -109,9 +109,13 @@ For containerized environments, the basic {{agent}} flavor is installed using th
 
 For containerized environments, the servers {{agent}} flavor is installed using the default `elastic-agent` command with an agent container package. This flavor contains the same set of components described in [Servers flavor](#elastic-agent-servers-flavor-regular) above.
 
+{applies_to}`stack: ga 9.1` The servers {{agent}} flavor also includes the [journald](https://www.freedesktop.org/software/systemd/man/latest/systemd-journald.service.html) dependencies necessary to use the [journald input](beats://reference/filebeat/filebeat-input-journald.md).
+
 #### Complete flavor [elastic-agent-complete-flavor]
 
-For containerized environments, the complete {{agent}} flavor is installed using the `elastic-agent-complete` command with an agent container package. This flavor includes all of the components in the servers flavor, and also includes additional dependencies to run browser monitors through Elastic Synthetics. It also includes the [journald](https://www.freedesktop.org/software/systemd/man/latest/systemd-journald.service.html) dependences necessary to use the [journald input](beats://reference/filebeat/filebeat-input-journald.md). Refer to [Synthetic monitoring via Elastic Agent and Fleet](/solutions/observability/synthetics/get-started.md) for more information.
+For containerized environments, the complete {{agent}} flavor is installed using the `elastic-agent-complete` command with an agent container package. This flavor includes all of the components in the servers flavor, and also includes additional dependencies to run browser monitors through Elastic Synthetics. Refer to [Synthetic monitoring via Elastic Agent and Fleet](/solutions/observability/synthetics/get-started.md) for more information.
+
+The complete {{agent}} flavor also includes the [journald](https://www.freedesktop.org/software/systemd/man/latest/systemd-journald.service.html) dependencies necessary to use the [journald input](beats://reference/filebeat/filebeat-input-journald.md).
 
 ## Resource requirements [elastic-agent-installation-resource-requirements]