Change to using a single date formatter and update unit tests.

[cprince-foreflight]

#284

As described in the ticket above, the two date formatters in the library didn't match up in the strings they produce. I've consolidated to a single date formatter using the string format like:
"2023-12-14T16:03:14.300Z"

The format used by the other date formatter, that I've removed, was:
"2023-12-13T23:26:37.091+0000"

Testing

• Changed a unit test to support this single date formatter.
• Did some initial testing in our own app by bringing the changed analytics-swift package in and looking at Segment debugger output for dates with Codable and non-Codable interfaces. Both now are the same in my tests.

[bsneed]

Released as 1.5.1