[Filebeat] journald: Make include_matches backwards compatible with 7.x config

[andrewkroh]

What does this PR do?

To make transitioning from 7.x/8.0 to 8.1 easier allow the old format to continue working
by translating it internally to the new format. This logs a deprecation warning when the
old config format is detected.

Why is it important?

This will make it easier for Agent users to migrate from 7.x/8.0 to 8.1. There will be time window
when integrations with the old format are running with the new Agent.

Checklist

• 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.

Related issues

• Relates [8.0] iptables system test: found 0 hits in logs-iptables.log-ep data stream integrations#2602
• Relates Add more filtering options to journald input #29294 (comment)

Logs

{
  "@timestamp": "2022-01-26T17:26:47.443Z",
  "ecs.version": "1.6.0",
  "log.level": "warn",
  "log.logger": "cfgwarn",
  "log.origin": {
    "file.line": 97,
    "file.name": "journald/config.go"
  },
  "message": "DEPRECATED: Please migrate your journald input's include_matches config to the new more expressive format.",
  "service.name": "filebeat"
}

[mergify]

This pull request does not have a backport label. Could you fix it @andrewkroh? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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

[elasticmachine]

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Build stats

• Start Time: 2022-01-27T01:02:03.442+0000

• Duration: 22 min 58 sec

• Commit: 7031342

❕ Flaky test report

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[andrewkroh]

/test