sboms: Update changelog and improve error message

[willdollman]

Add missing changelog entry for #1115.

Also improve the error message when an unsupported version is passed:

Unsupported version <=5.8.0:

> go run ./cmd/src sbom fetch -v 5.8.0 --internal --insecure-ignore-tlog
unsupported version 5.8.0: SBOMs are only available for Sourcegraph releases after 5.8.0

Supported version >5.8.0:

>  go run ./cmd/src sbom fetch -v 5.8.287 --internal --insecure-ignore-tlog
Fetching SBOMs and validating signatures for all 55 images in the Sourcegraph 5.8.287 release...

⚠️ WARNING: Transparency log verification is disabled, increasing the risk that SBOMs may have been tampered with.
️          This setting should only be used for testing or under explicit instruction from Sourcegraph.

✅ us-central1-docker.pkg.dev/sourcegraph-ci/rfc795-internal/appliance
✅ us-central1-docker.pkg.dev/sourcegraph-ci/rfc795-internal/batcheshelper
[...]

Test plan

• Tested change locally
• CI