Fix broken documentation links in attestations.go

The documentation links for the AttestationsService interface are currently failing validation. The specific anchors (#list-all-attestations and #download-an-attestation) appear to have changed in the upstream GitLab API documentation

Proposed Solution:

Update the documentation links to point to the main Attestations API landing page to ensure users reach the correct documentation while upstream anchor naming is stabilized.

Affected Links:

https://docs.gitlab.com/api/attestations/#list-all-attestations -> https://docs.gitlab.com/api/attestations

https://docs.gitlab.com/api/attestations/#download-an-attestation -> https://docs.gitlab.com/api/attestations