Support both tzinfo v1 and v2 alongwith non-half hour offsets.

[philr]

This is a 🙋 feature or enhancement.

Summary

This pull request changes WinTZ.calculate to support use with tzinfo v2 as well as the currently supported v1. Instead of determining the offset from the difference between local time and UTC, it now uses the defined offset returned by tzinfo (in a v1 and v2 compatible way).

The offset minutes calculation has also been changed to allow for time zones that don't use hour or half hour offsets. For example, Australia/Eucla's is at +08:45. Before the change, this would have resulted in WTZ-08:30 instead of the correct WTZ-08:45.

The new gemfile_contents and Windows documentation have updated to allow use with either tzinfo v1 or v2.

Unit tests for WinTZ have been added. This required adding tzinfo and tzinfo-data to the :test group in the Gemfile.

Context

Resolves #8516.

[ashmaroli]

@philr Thank you very much for submitting this PR.
Everything LGTM on a quick read-through. If its not asking too much, could you please add a Cucumber based test affirming the non-half hour offset behavior in features/site_configuration.feature.

This module is currently used only for builds on Windows platform. So, does native zoneinfo on non-Windows OS / platforms correctly set non-half hour offset out-of-the-box? If not, could the WinTZ module be re implemented as a platform-agnostic TZ handler? (Mainly asking w.r.t a long-existing issue that got closed due to inactivity; #7550)

[philr]

I've added a scenario to test the non-half hour offset behaviour.

Native zoneinfo will handle the non-half hour offset correctly.

[mattr-]

LGTM

[ashmaroli]

@mattr- Requesting a re-review for this one.
I updated the branch to:

• use tzinfo-data gem and tzinfo gem only on Windows (and JRuby)
• additional Appveyor test-matrix entries to explicitly test with tzinfo v1 and tzinfo v2. (Downside being increased CI runtime).

[philr]

I moved tzinfo into the test group because the new TestWinTz tests run on all platforms. The tests are still passing on non-Windows because tzinfo is being brought in through another dependency.

[ashmaroli]

The WinTZ utility module is only used in Windows:

jekyll/lib/jekyll.rb

Lines 133 to 139 in f51ccbf

	def set_timezone(timezone)
	ENV["TZ"] = if Utils::Platforms.really_windows?
	Utils::WinTZ.calculate(timezone)
	else
	timezone
	end
	end

So as of now, testing the module on non-Windows platforms is redundant.
That said, I was hoping to use the WinTZ in all platforms in a separate PR, after merging this.

[ashmaroli]

Totally forgot about this.
Thank you very much @philr. 🙏

@jekyllbot: merge +minor

[parkr]

Backporting to v3.9.x in #9280.