Check for package-feeds pubsub `SchemaVer` field in Scheduler

[tom--pollard]

The package-feeds project now validates & versions its package data output against a json schema https://github.com/ossf/package-feeds/blob/main/package.schema.json

The SchemaVer should follow semantic versioning, as such any minor 1.* release should guarantee compatibility for the current Scheduler expectations (Name, Version, Type). Any major version bump could signal incompatability (see https://github.com/ossf/package-feeds/issues/93 for example) as such this value should be checked before blindly accessing the current fields

[oliverchang]

Thanks for the head's up!