Bump actions/setup-python from 4 to 6

Bumps [actions/setup-python](https://github.com/actions/setup-python) from 4 to 6.
- [Release notes](https://github.com/actions/setup-python/releases)
- [Commits](actions/setup-python@v4...v6)

---
updated-dependencies:
- dependency-name: actions/setup-python
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/mac.yml

@@ -48,7 +48,7 @@ jobs:
     - uses: actions/checkout@v6
       with:
         path: ws/src/trajopt
-    - uses: actions/setup-python@v4
+    - uses: actions/setup-python@v6
       id: setup-python
       with:
         python-version: '3.12'