fix: Juju passes the expiry in a field 'expiry', not 'expires'

[tonyandrewmeyer]

secret-info-get gives the expiry back in a field called "expiry":

$ juju exec --unit thirteensixteen/0 -- secret-info-get cqv88thtvhl393c8hs20
cqv88thtvhl393c8hs20:
  revision: 1
  label: food
  owner: application
  rotation: daily
  expiry: 2024-09-04T23:12:09Z
  rotates: 2024-08-16T23:12:10Z

We were incorrectly looking for it in a field called "expires", which is the attribute name we use.

Fixes #1316

[benhoyt]

Thanks! Oddly, though the JSON/YAML field is called expiry, the Go field name is LatestExpireTime: https://github.com/juju/juju/blob/55ca71ee96903fbcbf45c2f1fa2326d14173152a/worker/uniter/runner/jujuc/secret-info-get.go#L83