Fixes marian-code#19 Updating README and adding an example for use-external-python

The macOS 13 runner is retired.

Update macOS runner from macOS 13 to macOS 14

Fix an oversight from ab76874
where the matrix was not properly updated to exclude older Python.

Fix an oversight from ab76874
where the matrix was not properly updated to exclude older Python.

Details:
 * Update actions/checkout from v4 to v6
 * Update actions/setup-python from v5 to v6

https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

marian-code/python-lint-annotate@fd01666
broke setting the Python version for the action.

While use-external-python works in
.github/workflows/test-python-install.yml, it does not work in the
action itself. This is because custom action inputs cannot use true
booleans, so the use-external-python input is a string type. This string
then always gets interpreted as "true", so the "Setup python" condition
always resolves to "false" due to negation.

Use fromJSON to correctly convert the strings "true" and "false" to
boolean types.

This also raises the minimum required Python version to 3.10, so update
the supported Python version range to 3.10-3.14.

Note that Python 3.10 will not install correctly on macOS 14+, so it is
still excluded from those runners.

Fix macOS matrix excludes

Fix macOS matrix, update first-party GitHub Actions, fix use-external-python, update all requirements and linters