Remove the default allowed audience in gh-oidc

[yu-iskw]

Overview

The official version of GitHub OpenID connection no longer requires audience=sigstore. So, the default value of allowed_audiences can be an empty list.
https://docs.github.com/ja/actions/deployment/security-hardening-your-deployments/configuring-openid-connect-in-google-cloud-platform

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[comment-bot-dev]

Thanks for the PR! 🚀
✅ Lint checks have passed.

[bharathkkb]

Thanks for the PR @yu-iskw
We just need to update the tests here

terraform-google-github-actions-runners/test/integration/oidc-simple/oidc_simple_test.go

Lines 37 to 38 in 76115f9

	assert.Equal(1, len(provider.Get("oidc.allowedAudiences").Array()), "WI provider has correct number of audiences")
	assert.Equal("sigstore", provider.Get("oidc.allowedAudiences").Array()[0].String(), "WI provider has correct audience")

[yu-iskw]

Ok. I will fix it.