Deprecate lenient RFC 3339 date time separators

[youdie006]

Follow-up to ruby/time#85, where you specified the approach:

To detect the invalid character, in the rfc3339 static function, add a capture group for the separator ("(?:t|\\s)" -> "(t|\\s)"), and in the rfc3339_cb static function, issue the warning if the capture group character doesn't equal t, T, or .

Date.rfc3339, DateTime.rfc3339 and Date._rfc3339 accept seven separators: T, a space, and \t\n\v\f\r. This warns on the last five under Warning[:deprecated], changing nothing else, so they can become Date::Error later.

Warning[:deprecated] = true
Date._rfc3339("2001-02-03\t04:05:06Z")
# warning: Invalid RFC3339 date time separator provided, this will raise an error in a future version.

Adding the capture group shifts the remaining groups, so SNUM goes 8 to 9 and the hour/min/sec/sec_fraction/zone indices move up by one.

One deviation from your suggestion

rb_warning_category_enabled_p is not usable from an extension: it is in no public header, and on 3.4 it is not exported from libruby at all (nm -D libruby.so | grep -c rb_warning_category_enabled_p is 0). Building with it fails on ruby 2.7 ('RB_WARN_CATEGORY_DEPRECATED' undeclared) and on ruby 3.4 (implicit declaration of function).

So the warning goes through rb_category_warn, which is public and which ext/date/extconf.rb:10 already probes, with the #ifndef HAVE_RB_CATEGORY_WARN fallback date_core.c:4690-4692 already uses so the gemspec's 2.6 floor keeps building. That also makes the outer guard redundant: rb_category_warn reports only when $VERBOSE is non-nil and the category is enabled. Happy to change it if you had something else in mind.

Verification

Date._rfc3339 parses identically for all seven separators in every row; only the warning is new.

row	result
new test, fix applied	2 tests, 19 assertions, 0 failures
new test, date_parse.c reverted	1 failure
warn unconditionally (drop the t/T/space check)	1 failure, the T/t/space rows
keep the capture group but leave hour reading s[4]	2 failures
revert (t|\s) to (?:t|\s), keep the renumbering	2 errors

suite	result
full test/date, ruby 2.7.8	148 tests, 162607 assertions, 0 failures
full test/date, ruby 3.2.11	148 tests, 162619 assertions, 0 failures
full test/date, ruby 3.4.10	148 tests, 162619 assertions, 0 failures
test_date_parse.rb, truffleruby 3.3.7	28 tests, 928 assertions, 0 failures (1 failure before the fix)

With deprecation warnings off, which is the default, nothing is emitted.

I kept this on its own branch so it stays independent of #190.

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Disclosure: I used Claude (an AI assistant) while preparing this change. Every result above I ran and verified myself.