Use checkpoint_log_id instead of log_id

[loosebazooka]

log_id is considered deprecated: https://github.com/sigstore/protobuf-specs/blob/main/protos/sigstore_trustroot.proto#L54.

Since this is from parsing the trusted_root and not entries/bundles themselves, we can move off of log_id without breaking backward compat? @haydentherapper check me on this?

[haydentherapper]

This might have been premature to mark as deprecated, I didn't realize how many clients' linters would complain.

You can keep using log ID, we'll populate both checkpoint key ID and log ID going forward. For Rekor v2 logs, these should be the same. And yes, you should be able to move off log ID once checkpoint key ID is populated (we dont have a trusted root published yet with it populated).