Upgrade action/cache to v4

[rory-loughrey-kainos]

Deprecation of Node20 on Github Action runners

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

Identified that action/cache must be bumped to v5 to ensure compatability with Node 24

https://github.com/actions/cache/releases/tag/v5.0.0

[ohshazbot]

I see this ticket is closed but it's not actually completed. This isn't a feature, this is maintenance, node 20 will be removed at the end of the year (and will require an additional env variable to operate after June2).

[asottile]

no it won't. it will automatically work just like the last n times GitHub has done this

[whyscream]

I know the problem is harmless, and Github is causing extra work for you as a maintainer by this. But obviously the notice caused by GitHub scratches many peoples itches, which is what causes the duplicate bug reports and PRs (For every 10 ppl that notice the duplicate issue, there will always be 1 person that won't). This is the downside of opensource maintainership, I guess. Referring to #189 (comment) as well.

If a PR was made to resolve the issue (twice already in this case), it would be nice to merge it. For contributors who invest time, for all the people who get itched by a warning/notice they cannot resolve themselves, and for yourself: repeatedly closing issues and PRs for something that is easily resolvable must be annoying too.

[immoseb]

@asottile could closing #242 and #244 be reconsidered please? I don't see how they hurt.