Update slsa parser to remove deprecated structs

[Yaxhveer]

Description of the PR

Fixes #1987

Updated the slsa parser by removing deprecated structs of in-toto-golang with the in-toto attestation repo. Added nil pointer checks wherever needed.

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If GraphQL schema is changed, GraphQL client updates/additions have been made
• If OpenAPI spec is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[mihaimaruseac]

Thank you!

[pxp928]

Thanks @Yaxhveer! Added a few comments but overall looks good.

[pxp928]

LGTM! Great work