[SINT-4729] use dd-octo-sts in reusable-integration-test workflow

[ikraemer-dd]

Summary

• Migrate reusable-integration-test.yml from GitHub App token (actions/create-github-app-token) to dd-octo-sts OIDC-based token (DataDog/dd-octo-sts-action)
• Move permissions from workflow-level to job-level, adding id-token: write for OIDC
• Part of splitting [SINT-4729] use dd-octo-sts policies #3477 into per-file PRs

Changes

• Replace actions/create-github-app-token@v1 with DataDog/dd-octo-sts-action@acaa02eee7e3bb0839e4272dacb37b8f3b58ba80 (v1.0.3)
• Use scope: DataDog/${{ inputs.target-repo || 'datadog-api-spec' }} and policy: datadog-api-client-java.reusable-integration-test.post-status
• Remove top-level permissions block, add job-level permissions with contents: read and id-token: write
• Remove PIPELINE_GITHUB_APP_ID and PIPELINE_GITHUB_APP_PRIVATE_KEY from the secrets input declarations