2.0: Documentation updates for COSIGN_EXPERIMENTAL and new flags

[haydentherapper]

Description

We need to remove references to COSIGN_EXPERIMENTAL in code (tests and CLI docs), documentation, and blog posts. We also need to update documentation with any new required flags (like identity flags) and optional flags (like tlog-upload).

[haydentherapper]

Flag documentation also includes [EXPERIMENTAL] in many places, that needs to be removed.

[znewman01]

Timing is a little awkward here. There's a few changes we should have staged up and ready to go simultaneous with the release (anything breaking). Before the 2.0 release we can't push most of these.

Leaving COSIGN_EXPERIMENTAL in for a while doesn't actually hurt anything, and I worry switching too soon will cause confusion before folks have updated.

[haydentherapper]

All examples in the code should reflect the current state imo, so we should at least remove the references to experimental in the code base.

We also need to go add the newly required identity flags, since all existing example commands won’t work.