runtime.txt is deprecated

[lotas]

remote: ! Warning: The runtime.txt file is deprecated.
remote: !
remote: ! The runtime.txt file is deprecated since it has been replaced
remote: ! by the more widely supported .python-version file:
remote: ! https://devcenter.heroku.com/changelog-items/3141
remote: !
remote: ! Please switch to using a .python-version file instead.
remote: !
remote: ! Delete your runtime.txt file and create a new file in the
remote: ! root directory of your app named:
remote: ! .python-version
remote: !
remote: ! Make sure to include the '.' character at the start of the
remote: ! filename. Don't add a file extension such as '.txt'.
remote: !
remote: ! In the new file, specify your app's major Python version number
remote: ! only. Don't include quotes or a 'python-' prefix.
remote: !
remote: ! For example, to request the latest version of Python 3.13,
remote: ! update your .python-version file so it contains exactly:
remote: ! 3.13
remote: !
remote: ! We strongly recommend that you don't specify the Python patch
remote: ! version number, since it will pin your app to an exact Python
remote: ! version and so stop your app from receiving security updates
remote: ! each time it builds.
remote: !
remote: ! In the future support for runtime.txt will be removed and
remote: ! this warning will be made an error.