symantec_endpoint: use valid ECS values for fields

[efd6]

What does this PR do?

Changes values assigned to ECS field to conform to the spec.

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

• Closes symantec_endpoint using invalid field values according to ECS #3050

Screenshots

[elasticmachine]

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

[r00tu53r]

LGTM

[elasticmachine]

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• Start Time: 2022-05-02T01:34:03.324+0000

• Duration: 18 min 44 sec

Test stats 🧪

Test	Results
Failed	0
Passed	23
Skipped	0
Total	23

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚 2.857
Classes	100.0% (1/1)	💚 2.857
Methods	100.0% (19/19)	💚 10.947
Lines	98.914% (820/829)	👍 8.881
Conditionals	100.0% (0/0)	💚