Remove experimental warning from journald input.

[cmacknz]

Mark the journald input as GA now that it is used as part of the system module and Elastic Agent system integration to collect system logs on Linux distributions that do not enable syslog by default.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @cmacknz? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[belimawr]

You still need to update the "stability" of the input:

beats/filebeat/input/journald/input.go

Line 76 in 3cdd856

Stability: feature.Experimental,

That forces some deprecation logs. I believe that's the only place.

[cmacknz]

Thanks for catching that, updated.

[cmacknz]

Removing backport to 8.17 until elastic/integrations#9690 is addressed.

[flexitrev]

Looks good