Update references to GitHub actions

[ckunki]

Summary

GitHub reports

Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: actions/cache@v4, actions/setup-python@v5. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026. Please check if updated versions of these actions are available that support Node.js 24. To opt into Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment variable on the runner or in your workflow file. Once Node.js 24 becomes the default, you can temporarily opt out by setting ACTIONS…

PTB action.yaml uses actions/cache@v4 and actions/setup-python@v5.

There are newer versions https://github.com/actions/cache/releases/tag/v5.0.3 and https://github.com/actions/setup-python/releases/tag/v6.2.0.

So I guess we should soon update the PTB to use newer version, which hopefully are backwards-compatible.