Update dependency icalendar to v6

This MR contains the following updates:

Package	Type	Update	Change
icalendar (changelog)	dependencies	major	5.0.13 -> 6.1.0

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Release Notes

collective/icalendar (icalendar)

v6.1.0

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Minor changes:

• Add end, start, duration, DTSTART, DUE, and DURATION attributes to Todo components. See Issue 662_.
• Add DTSTART, TZOFFSETTO and TZOFFSETFROM properties to TimezoneStandard and TimezoneDaylight. See Issue 662_.
• Format test code with Ruff. See Issue 672 <https://github.com/collective/icalendar/issues/672>_.
• Document the Debian package. See Issue 701 <https://github.com/collective/icalendar/issues/701>_.
• Document vDatetime.from_ical
• Allow passing a datetime.date to TZP.localize_utc and TZP.localize methods.
• Document component classes with description from :rfc:5545.
• Merge "File Structure" and "Overview" sections in the docs. See Issue 626 <https://github.com/collective/icalendar/issues/626>_.
• Update code blocks in usage.rst with the correct lexer.
• Tidy up the docstring for icalendar.prop.vPeriod.
• Improve typing and fix typing issues

New features:

• Add VALARM properties for :rfc:9074. See Issue 657 <https://github.com/collective/icalendar/issues/657>_
• Test compatibility with Python 3.13
• Add Timezone.from_tzinfo() and Timezone.from_tzid() to create a Timezone component from a datetime.tzinfo timezone. See Issue 722_.
• Add icalendar.prop.tzid_from_tzinfo.
• Add icalendar.alarms module to calculate alarm times. See Issue 716 <https://github.com/collective/icalendar/issues/716>_.
• Add Event.alarms and Todo.alarms to access alarm calculation.
• Add Component.DTSTAMP and Component.LAST_MODIFIED properties for datetime in UTC.
• Add Component.is_thunderbird() to check if the component uses custom properties by Thunderbird.
• Add X_MOZ_SNOOZE_TIME and X_MOZ_LASTACK properties to Event and Todo.
• Add Alarm.ACKNOWLEDGED, Alarm.TRIGGER, Alarm.REPEAT, and Alarm.DURATION properties as well as Alarm.triggers to calculate alarm triggers.
• Add __doc__ string documentation for vDate, vBoolean, vCalAddress, vDuration, vFloat, vGeo, vInt, vPeriod, vTime, vUTCOffset and vUri. See Issue 742 <https://github.com/collective/icalendar/issues/742>_.
• Add DTSTART, TZOFFSETTO, and TZOFFSETFROM to TimezoneStandard and TimezoneDaylight
• Use example methods of components without arguments.
• Add events, timezones, and todos property to Calendar for nicer access.
• To calculate which timezones are in use and add them to the Calendar when needed these methods are added: get_used_tzids, get_missing_tzids, and add_missing_timezones.
• Identify the TZID of more timezones from dateutil.
• Identify totally unknown timezones using a UTC offset lookup tree generated in icalendar.timezone.equivalent_timezone_ids and stored in icalendar.timezone.equivalent_timezone_ids.
• Add icalendar.timezone.tzid to identify a timezone's TZID.

Bug fixes:

• Add icalendar.timezone to the documentation.

.. _Issue 722: https://github.com/collective/icalendar/issues/722

v6.0.1

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New features:

• Added end, start, duration, DTSTART, DUE, and DURATION attributes to Event components. See Issue 662_.
• Added end, start, duration, and DTSTART attributes to Journal components. See Issue 662_.

Bug fixes:

• Fix a few __all__ variables.
• Added missing docs folder to distribution packages. See Issue 712 <https://github.com/collective/icalendar/issues/712>_.

.. _Issue 662: https://github.com/collective/icalendar/issues/662

v6.0.0

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Minor changes:

• Add __all__ variable to each modules in icalendar package
• Improve test coverage.
• Adapt test_with_doctest.py to correctly run on Windows.
• Measure branch coverage when running tests.
• Export Component base class for typing

New features:

• Use pyproject.toml file instead of setup.py

Bug fixes:

• Fix link to stable release of tox in documentation.
• Fix a bad bytes replace in unescape_char.
• Handle ValueError in vBinary.from_ical.
• Ignore the BOM character in incorrectly encoded ics files.

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