[github] change API key variable to optional for security advisories data stream

[brijesh-elastic]

Proposed commit message

github: change API key variable to optional for security advisories data stream,
as authentication is not required for public repositories

[1] https://docs.github.com/en/rest/security-advisories/global-advisories?apiVersion=2026-03-10#list-global-security-advisories

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.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

How to test this PR locally

• Clone integrations repo.
• Install elastic package locally.
• Start elastic stack using elastic-package.
• Move to integrations/packages/github directory.
• Run the following command to run tests.

elastic-package test -v

Related issues

• [Github] Make the PAT settings in the Github Security Advisories dataset optional #18318

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[github-actions]

Vale Linting Results

Summary: 2 suggestions found

💡 Suggestions (2)

File	Line	Rule	Message
packages/github/_dev/build/docs/README.md	116	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.
packages/github/docs/README.md	1030	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.

---

The Vale linter checks documentation changes against the Elastic Docs style guide.

To use Vale locally or report issues, refer to Elastic style guide for Vale.

[elastic-vault-github-plugin-prod]

🚀 Benchmarks report

To see the full report comment with /test benchmark fullreport

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: f51e9d0

cc @brijesh-elastic