GitHub Actions workflow upkeep

[sajith]

Each run of GitHub Actions with the current configuration emits many warnings about deprecations. Consider this workflow run, for example:

​https://github.com/tahoe-lafs/tahoe-lafs/actions/runs/3533138743

There are a 100 warnings, mostly duplicates. The warnings are due to:

• Node.js 12 actions are deprecated. It seems that actions/checkout@v2, actions/setup-python@v2, actions/cache@v2, actions/upload-artifact@v2 all use older Node.js. They are all probably due for an upgrade.
• set-output and save-state commands are deprecated and will be disabled soon. This is from the pip caching steps; we can simply remove that and use the equivalent feature available in newer version of `actions/setup-python.

While we are on this, it probably is a good idea to stick this incantation in the workflow file so that the GITHUB_TOKEN issued at the beginning of the workflow will have a minimum set of permissions:

permissions: 
  contents: read

Relevant GitHub docs are ​Automatic token authentication and ​Workflow syntax for GitHub Actions: permissions.

I understand that GH Actions do not get a lot of love from Tahoe-LAFS team, and some of us would love to throw the thing away entirely. However, as long as Tahoe-LAFS uses GH Actions, it is a good idea to do the minimum upkeep.

[sajith]

​GitHub Actions: All Actions will begin running on Node16 instead of Node12 is relevant:

Node 12 has been out of support since April 2022, as a result we have started the deprecation process of Node 12 for GitHub Actions. We plan to migrate all actions to run on Node16 by Summer 2023. We will monitor the progress of the migration and listen to the community for how things are going before we define a final date. To raise awareness of the upcoming change, we are adding a warning into workflows which contain Actions running on Node 12. This will come into effect starting on September 27th.

[GitHub <noreply@…>]

In 7cc023a/trunk:

Merge pull request #1229 from tahoe-lafs/3944.github-actions-updates

GitHub? Actions updates

Fixes: ticket:3944