3.0.0-RC1

Merge pull request #11 from horde/refactor/strftime-icu-migration
refactor(strftime): Migrate to native usage of IntlDateFormatter and DateTime
chore: Add keywords, use PHP 8.1+
test: Cover HordeLegacyDate compatibility with Horde_Date
docs: update UPGRADING guide to .md format and strategies for adopting modern 3.0
feat: interface-compatible drop-in replacement for Horde_Date
style: php-cs-fixer in lib
style: tests, php-cs-fixer
style: php-cs-fixer
chore: Add Torben. Jan isn't currently active.
test: Amend recurrence tests
feat: Recurrence needs to mind timezones
test: Cover src/ variant with tests
feat: Flesh out Horde_Date src/ branch
fix: Casting and coupling problems
test: Improve coverage of recurrence, repeater, timezonealias
fix setlocale to provide proper type