build(deps): bump actions/setup-python from 5 to 6

Bumps [actions/setup-python](https://github.com/actions/setup-python)
from 5 to 6.
- [Release notes](https://github.com/actions/setup-python/releases)
- [Commits](https://github.com/actions/setup-python/compare/v5...v6)

Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index ea1d959..fa7c60f 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -259,7 +259,7 @@
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - uses: actions/checkout@v5
-    - uses: actions/setup-python@v5
+    - uses: actions/setup-python@v6
     - name: Set up dependencies
       shell: pwsh
       run: pip install meson ninja
@@ -287,7 +287,7 @@
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - uses: actions/checkout@v5
-    - uses: actions/setup-python@v5
+    - uses: actions/setup-python@v6
     - name: Set up dependencies
       shell: pwsh
       run: pip install meson ninja