[Filebeat] Bring syslog input to GA

[mostlyjason]

The syslog input on Filebeat is currently experiemental and it logs the following line:
2021-03-17T12:20:20.975+0100 WARN [cfgwarn] syslog/input.go:111 EXPERIMENTAL: Syslog input type is used

However, it is not communicated as experimental in the documentation. https://www.elastic.co/guide/en/beats/filebeat/7.11/filebeat-input-syslog.html

After discussing with engineers the reason it's experimental is the lack of RFC5424 and multiline support. We could document these limitations instead of letting them block the whole module from going GA.

The request for this issue is to remove the experimental log line, add the limitations to the docs, and make the input GA.

CC @mukeshelastic @andresrc @ph

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[jamiehynds]

We discussed this among the security integrations team earlier this week, and would love to move syslog to GA.

Just an FYI, we had a community contribution for RFC5424 support recently, waiting on review. #23954

[ph]

• Remove Experimental for RFC31*
• Add Beta for RFC5424
• Add Beta mention for RFC5424 in the documentation.

[urso]

The work in progress meta issue with open tasks is here: #24954

Can we close one of the two?