[journald] Add 8.x support

[andrewkroh]

What does this PR do?

Update kibana.constraint to allow 8.1.0+. 8.1.0 is required because elastic/beats#30032
is needed due to a breaking change that was introduced in 8.0.0.

Rename message_id to ECS event.code. This field was added in 8.0. From man systemd.journal-fields

MESSAGE_ID=
A 128-bit message identifier ID for recognizing certain
message types, if this is desirable. ...

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

[elasticmachine]

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

[elasticmachine]

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• Start Time: 2022-08-18T18:22:49.324+0000

• Duration: 12 min 19 sec

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Test	Results
Failed	0
Passed	3
Skipped	1
Total	4

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (0/0)	💚
Files	100.0% (0/0)	💚 2.825
Classes	100.0% (0/0)	💚 2.825
Methods	75.0% (3/4)	👎 -14.336
Lines	100.0% (0/0)	💚 9.089
Conditionals	100.0% (0/0)	💚

[cmacknz]

Thanks for taking care of this!