docs: Fix broken documentation links in attestations.go

Mohamed Othmanrequested to merge
gitlab-community/gitlab-org/api/client-go:fix-attestations-file-links into main

This MR addresses broken link errors identified in attestations.go. The previous anchors were no longer resolving correctly on the GitLab documentation site.

Changes:

Updated ListAttestations link to point to: https://docs.gitlab.com/api/attestations/#list-all-attestations

Updated DownloadAttestation link to point to https://docs.gitlab.com/api/attestations/#download-an-attestation.

Related issue: #2230 (closed)

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