Add the upgrade guide for v1.3

[JonasPardon]

Adds the v1.2 → v1.3 upgrade guide, one of the open tasks on wintercms/winter#1366, and bumps the requirements on the installation page to match.

The guide follows the format of the 1.2 guide. It covers the Winter changes and the parts of Laravel 10–12, Symfony 7, Carbon 3, Monolog 3 and PHPUnit 11 that reach Winter projects. Laravel changes that don't apply to Winter (such as the new application structure) are left out.

It is based on porting a large production Winter 1.2 project to storm 618d9fd / winter 0ae3104. The claims were checked against that code, and most were reproduced on the Laravel 12 install. Anything that changes before the release (for example the ~1.3.0 constraints, which assume a tagged release) will need a follow-up.

I've posted the bugs I found in wip/1.3 separately on wintercms/winter#1366.

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