Switch to offline Sigstore verification and set TimestampVerificationData to nil

[rdimitrov]

The following PR has the following changes:

• Recreates Switch to offline Sigstore verification #2120 switching Minder to using offline sigstore verification. For those that are interested there's a nice and detailed explanation of how this works at Switch to offline Sigstore verification #2120 (comment)
• Sets the TimestampVerificationData to nil. The reason is it was wrongfully used, but also its value is obtained from the SignedEntryTimestamp assuming verifyWithObserverTimestamps() is set (we do set it).

Thanks to @haydentherapper for raising this! 🍻

[haydentherapper]

Looks great!

[jhrozek]

formally rubber-stamping since both @haydentherapper approved this PR earlier but also @rdimitrov asked his original PR earlier.