Cherry-pick #18447 to 7.x: changed input from syslog to tcp/udp due to unsupported RFC

[adriansr]

Cherry-pick of PR #18447 to 7.x branch. Original message:

What does this PR do?

Changes beat input in docs and configuration file due to unsupported RFC syslog pattern.

Why is it important?

Module needs this fix to work properly.

Checklist

Added the changes, updated the documentation, ran nosetests and confirmed by testing with netcat to simulate the syslog input.

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

💚 Build Succeeded

Expand to view the summary

Build stats

• Build Cause: [Pull request Cherry-pick #18447 to 7.x: changed input from syslog to tcp/udp due to unsupported RFC #18527 opened]

• Start Time: 2020-05-14T15:22:00.067+0000

• Duration: 67 min 40 sec (4000232)

Test stats 🧪

Test	Results
Failed	0
Passed	2122
Skipped	290
Total	2412

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)