2.x - Update source files path.

Bump up phpstan.

Add microsecond() setter

Full french doc translation update

Update README.md

Add `ReturnTypeWillChange` annotation.

This prevents deprecation errors on PHP 8.1.

Use action/cache v2

Pipe cs output to cs2pr

Improve Carbon Migration Guide in Readme

CarbonImmutable instances should be converted to Chronos instance (which are immutable), rather than the non-existant "ChronosImmutable".

Fix diffInMonthsIgnoreTimezone() using wrong timezone

Fix int/timestamp support in Chronos constructor

Use actions/checkout v2

Add link annotations to RFCs

RFCs are important sources of formats and protocols.
Let's include them in the docs this way.

Cleanup diffInMonthsIgnoreTimezone and tests

Fix problem matcher

Add diffInMonthsIgnoreTimezone()

Add diff() that ignores timezone