checkpoint: fix src_user_name field mapping

[efd6]

What does this PR do?

Fixes mapping of the Check Point src_user_name field to source.user.name and removes redundant src_user_name documentation.

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.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Updates [docs] Fix wrong Check Point fields in the documentation #5002

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[elasticmachine]

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Build stats

• Start Time: 2023-02-20T00:13:48.506+0000

• Duration: 17 min 53 sec

Test stats 🧪

Test	Results
Failed	0
Passed	12
Skipped	0
Total	12

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚
Classes	100.0% (1/1)	💚
Methods	100.0% (16/16)	💚
Lines	85.805% (810/944)	❕
Conditionals	100.0% (0/0)	💚

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[kcreddy]

LGTM 👍🏼

[elasticmachine]

Package checkpoint - 1.15.1 containing this change is available at https://epr.elastic.co/search?package=checkpoint