feat: ignore non-channel version discovery docs

[noahdietz]

Ignore Discovery documents that do not use a channel-based version value. This still allows the pseudo-channel-based versions like beta and foo_v1 that exist today.

Fixes http://b/475855127

[noahdietz]

Need to update this to also exclude the non-CBV versions from the index.

[noahdietz]

Need to update this to also exclude the non-CBV versions from the index.

Realized we only needed to exclude non-CBV versions from the index and they would subsequently be excluded from the actual generated files.