📝 Update upgrade guide (#469)

## Description

This PR updates the upgrade guide to improve the reasoning for the drop
of support for x86 macOS systems.

## Checklist:

- [x] The pull request only contains commits that are focused and
relevant to this change.
- [x] ~I have added appropriate tests that cover the new/changed
functionality.~
- [x] ~I have updated the documentation to reflect these changes.~
- [x] ~I have added entries to the changelog for any noteworthy
additions, changes, fixes, or removals.~
- [x] ~I have added migration instructions to the upgrade guide (if
needed).~
- [x] The changes follow the project's style guidelines and introduce no
new warnings.
- [x] The changes are fully tested and pass the CI checks.
- [x] I have reviewed my own code changes.

Author: denialhaag

UPGRADING.md

@@ -8,7 +8,6 @@ This document describes breaking changes and how to upgrade. For a complete list
 
 Starting with this release, MQT Predictor no longer supports x86 macOS systems.
 This step is necessary to ensure compatibility with PyTorch.
-Furthermore, this coincides with GitHub removing the `macos-13` runners from their infrastructure.
 x86 macOS systems are no longer tested in our CI.
 
 ## [2.3.0] - 2025-07-29