[fortinet] Remove experimental label from manifest

[marc-gr]

What does this PR do?

Changes experimental label to GA, as the package is already in v1+ and the original intent is for it to be GA.

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.
• If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

[elasticmachine]

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

[elasticmachine]

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• Start Time: 2021-08-02T14:20:26.905+0000

• Duration: 31 min 24 sec

• Commit: ca1ac97

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Test	Results
Failed	0
Passed	28
Skipped	0
Total	28

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