Improved message for meeting previous and current thresholds

[haydentherapper]

Context PR: sigstore/root-signing-staging#57

When a threshold is updated, both the previous threshold with the previous set of keys and the new threshold with the latest set of keys must be met. The current message is "Role root is not yet verified. It is signed by 1/1 (1/2) signers ", where the first ratio is the previous threshold, the second ratio is the new threshold. To make this clearer, it would be helpful to denote which of these ratios is the previous or current threshold. Maybe something like " role is not yet verified. Previous threshold met (1/1). Current threshold not yet verified (1/2)." Could also include the version, like "Previous threshold for version X met, current threshold for X+1 not".

[jku]

role is not yet verified. Previous threshold met (1/1). Current threshold not yet verified (1/2).

My goal with the messages has been to keep them as short as possible (I already find myself not reading them all the way and I bet others have the same problem): this has so far prevented improving this string... your suggestion is pretty good though, should fit on a single line. I'm not sure everyone immediately knows what threshold means here though.

Another option might be to move the "verified/not verified" to the title:

---

❌ root (not verified yet)

Previous threshold met (1/1). Current threshold not met (1/2). Signed by @jku
Still missing signatures from @-kommendorkapten, @-mnm678, @-repo-import, @-joshuagl
Signers can sign these changes by running tuf-on-ci-sign sign/staging-with-more-signers

[haydentherapper]

I like moving the status into the title, lets verifiers quickly glance to see the status.

[joshuagl]

Another option might be to move the "verified/not verified" to the title:

The suggestion here is a great. 👍