Add try_to_rfc3339 and to_iso8601, deprecate to_rfc3339

[pitdicker]

Split out from #1144.

RFC 3339 is not defined for dates with a year outside of 0..=9999 (like RFC 2822). ISO 8601 does support it.

DateTime::to_rfc3339 was a hybrid method, claiming to format a datetime as both valid RFC 3339 and ISO 8601. It would write invalid RFC 3339 strings if the year is out of range, which could not be parsed by DateTime::parse_from_rfc3339.

I propose to deprecate to_rfc3339, and replace it with two methods: try_to_rfc3339 and to_iso8601.

I did not make to_rfc3339 panic on out-of-range dates like to_rfc2822, to not break existing uses.
It is deprecated with the message: "Produces invalid data on years outside of the range 0..=9999. Use try_to_rfc3339() or to_iso8601 instead."

For DateTime::to_rfc3339_opts and the RFC3339 formatting item (used for "%+") I only made some documentation adjustments that they format the value an ISO 8601 representation when the year is out of range.

[codecov]

Codecov Report

Attention: Patch coverage is 96.07843% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 91.80%. Comparing base (3114597) to head (5b394dd).

❗ Current head 5b394dd differs from pull request most recent head 37f8e74. Consider uploading reports for the commit 37f8e74 to get more accurate results

Files	Patch %	Lines
src/datetime/mod.rs	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1331      +/-   ##
==========================================
- Coverage   91.80%   91.80%   -0.01%     
==========================================
  Files          40       40              
  Lines       18329    18360      +31     
==========================================
+ Hits        16827    16855      +28     
- Misses       1502     1505       +3     

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