feat: Migrate GitHub Actions from PAT to GitHub App authentication

[mbruzda-splunk]

Description

Migrate GitHub Actions workflows from using a Personal Access Token (GH_TOKEN_ADMIN) to GitHub App authentication via actions/create-github-app-token@v3. GitHub App tokens are scoped, short-lived, and auditable, providing better security and traceability compared to long-lived Personal Access Tokens.

Checklist

• README.md has been updated or is not required
• push trigger tests
• manual release test
• automated releases test
• pull request trigger tests
• schedule trigger tests
• workflow errors/warnings reviewed and addressed

Testing done

https://github.com/splunk/splunk-add-on-for-salesforce/actions/runs/24390045573