Make timestamp/established time policy enforcement clearer

[jku]

Currently our established time policy is a little weird and I'd like to to make it clear and correct:

• we don't (yet) have a way to define more complex policies like requiring more than one timestamp : this is fine
• The default policy is or should be "require at least one established time from a source listed in trusted_root.json" where established time is a TSA timestamp or alternatively a rekor v1 integrated time
• Currently there are two issues:
  • If the bundle contains TSA timestamps, then one of the TSA timestamps is ALSO required to be valid (even if the entry is rekorv1 entry and has a valid integrated time). This means an invalid bundle can be made valid by just removing a timestamp from it
  • If a rekor v2 entry did somehow contain an integrated time, this would be a valid established time for the time policy code ... (maybe this is not technically possible but I'd like to see it in the code )

The first issue currently prevents the following scenario

• sign bundle with --rekor-version 1 cli: Add --rekor-version to sign arguments #1471 This leads to a bundle with rekor v1 entry and a TSA timestamp
• verify bundle with previous release 3.6.4 (currently fails with not enough timestamps validated to meet the validation threshold (0/1) maybe because the timestamp signing key type was not yet supported?)

I don't know how important this is but it feels like a source of future issues.