bugfix: use correct time; fix brittle unit test

[jzacsh]

this is fixing a single time-mishandling issue that uncovered two orthogonal problems:

• 1. "correct time" issue: json parsing dropped timezone/offset specifiers as we were using a hand-crafted formatter string. Switching us to a standard library one that accepts both Z alias (for greenwhich) and offsets (like +00:00). Gphotos api docs[a] (inlined into the code) shows we use the former, which is what the unit test uses as well.
     • this probably meant time metadata for content transferred was off by some number of due to timezone being dropped
• 2. "brittle unit test": unit test was relying on machine-specific gradle test behavior, so an out of the box git checkout master had two failing tests for me[b] but not for our github ci/cd.
     • this was due to use of toString() (in our assertEquals) which relies on your locale
     • fixed now as we use java.time.Instant#equals instead.

[a]: https://developers.google.com/photos/library/reference/rest/v1/mediaItems#mediametadata lists creationTime as:

string (Timestamp format)

Time when the media item was first created (not when it was uploaded to Google Photos).

A timestamp in RFC3339 UTC "Zulu" format, with nanosecond resolution and up to nine fractional digits. Examples: "2014-10-02T15:01:23Z" and "2014-10-02T15:01:23.045123456Z".

[b]: local unit test failure looked like this for me (I'm in midwest US):

org.opentest4j.AssertionFailedError:
    expected: <Mon Oct 02 22:33:38 UTC 2023>
    but was: <Mon Oct 02 22:33:38 CDT 2023>