feat: reduce default importers to only those used in V1

[jcrossley3]

This will facilitate the transition of V1 users to V2.

Relates to #953

[ctron]

As this is never exposed to a v1 user, I am against merging this. To my understanding also @mid998 is relying on this sometimes.

[jcrossley3]

I just think the essence of the feature is better conveyed through a minimum of default examples. Thirteen importers is overwhelming to a new user unfamiliar with the feature. Worse, the only sample SBOM importer doesn't even appear on the first page!

IMO the 10 importers I removed belong in the docs as examples accompanied by supporting text descriptions of each field.

[ctron]

I agree that 10 importers might not be the best approach. On the other side, ripping out the existing other times leaves us without any example of using OSV, CWE, ClearlyDefined, … In order to compensate, we should improve docs to enable users to get there. Or, just stick to the existing importers, on which PM relies today.