docs: update to Laravel 13 and PHP 8.3 minimum

[imorland]

Summary

Updates the Flarum 2.0 documentation to reflect the Laravel 13 upgrade and PHP 8.3 minimum.

docs/extend/update-2_0.md (the 1.x → 2.x extension upgrade guide):

• PHP 8.2 → 8.3 minimum throughout
• Laravel section heading and body updated from 12.x to 13.x
• Inline upgrade guide link updated to 13.x
• Added Laravel 13 upgrade guide link to the "for more details" line (L9–L12 links kept — the guide covers the full journey from Flarum 1's Laravel 8)

docs/install.md:

• Server requirements: PHP 8.2+ → 8.3+
• Removed the four PHP 8.2 archive rows from the installation table (8.2 is no longer a supported minimum)

Related

• [2.x] chore: upgrade Illuminate components to Laravel 13 framework#4468