fortinet_fortigate: allow syslog lines without a priority

[efd6]

What does this PR do?

Allows syslog lines without a priority.

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 Fortigate Initial grok option to ignore syslog priority #6702

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Build stats

• Start Time: 2023-06-30T07:10:01.200+0000

• Duration: 17 min 53 sec

Test stats 🧪

Test	Results
Failed	0
Passed	9
Skipped	0
Total	9

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (4/4)	💚
Classes	100.0% (4/4)	💚
Methods	100.0% (40/40)	💚 39.241
Lines	91.386% (1167/1277)	👎 -8.15
Conditionals	100.0% (0/0)	💚

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

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Package fortinet_fortigate - 1.15.0 containing this change is available at https://epr.elastic.co/search?package=fortinet_fortigate