chore(actions): Add attestation URL, download and verify commands on release notes

[ViacheslavKudinov]

Resolves #ISSUE_NUMBER

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Before the change?

• Release notes do not have the link to attestation URL

After the change?

• ✅ The release notes include the link to attestation URL and commands how to download and verify release artifacts

Example: https://github.com/vk-or/terraform-provider-github/releases/tag/v0.0.18

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[ViacheslavKudinov]

Hi @nickfloyd
feel free to add into relevant milestone.

[stevehipwell]

This pattern means that release notifications wont contain the attestation text as it's appended after the release. My suggestion would either be to link to docs on how to validate or to use custom release notes.