Use always internal parsing for iso date/times

[elacour]

When using RT "unknown" date format, external perl modules are used and fails:
* Time::ParseDate fail to parse iso format and produce wrong date/time,
* DateTime::Format::Natural claims string isn't parseable.

This is a problem when updating RT DateTime CF from REST where iso8601 is recommended. _CanonicalizeValueDateTime parse input as "unknown" and produces wrong dates.

[elacour]

Before this PR:

$ ./test.pl
Testing with "2026-02-13T17:16:35Z"

Parsed as "unknown" with Time::ParseDate: 2026-02-12 23:00:00

[900183] [Fri Feb 13 17:38:23 2026] [warning]: Couldn't parse date '2026-02-13T17:16:35Z' by DateTime::Format::Natural
Parsed as "unknown" with DateTime::Format::Natural: 2026-02-12 23:00:00

Parsed as "iso": 2026-02-13 17:16:35

[elacour]

Maybe adding a chapter on REST2 doc about doc formats would be nice ?

[cbrandtbuffalo]

• Time::ParseDate fail to parse iso format and produce wrong date/time,

I'm inclined to try to get a fix in Time::ParseDate rather than covering up a bug in the RT code. There is actually a PR with a fix: muir/Time-modules#10, but the module is looking for a new maintainer. I'll look into that and see what the options are.