Update GitHub Actions to Node.js 24 runtimes

[sjoerddebruin]

CI currently warns on every run:

Node.js 20 is deprecated. The following actions target Node.js 20 but are
being forced to run on Node.js 24: actions/checkout@v4,
actions/setup-dotnet@v4, actions/upload-artifact@v4.

The runners are already forcing these onto Node 24, so this bumps each
Node-based action to a major version that targets node24 natively.

Action	Before	After
actions/checkout	v4	v7
actions/setup-dotnet	v4	v6
actions/upload-artifact	v4	v7
actions/download-artifact	v4	v8
softprops/action-gh-release	v2	v3
jiro4989/build-deb-action	v3	v4

download-artifact and action-gh-release weren't named in the warning
because the release job only runs on tags, but they were on Node 20 too.

build-deb-action is Docker-based so it wasn't part of the deprecation, but
v4 is current — it's reimplemented in Go and moves the base image from Ubuntu
22.04 to 24.04. jiro4989/build-rpm-action@v2 is already the latest release.

Claude Opus 5 reviewed the release notes for every major bump above; none of
the breaking changes apply to how this workflow uses these actions.

🤖 Generated with Claude Code

[Gymnasiast]

Not chuffed this PR uses AI, but the diff looks OK.