CI: six actions still on the deprecated Node 20 runtime

[MacBuchi]

Workflow runs log:

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.

The runner force-runs them on Node 24 today, so this is a warning — but the fallback is temporary (changelog). Once it is removed the affected steps fail outright.

The warning lists per job and names only actions/checkout, but six pinned actions resolve to a node20 runtime. Verified by reading each action's action.yml at the pinned ref:

action	pinned	runtime	latest	occurrences
actions/checkout	v4	node20	v7	bump.yml ×1, ci.yml ×3, release.yml ×1
actions/upload-artifact	v4	node20	v7	ci.yml ×1, release.yml ×1
actions/download-artifact	v4	node20	v8	release.yml ×1
softprops/action-gh-release	v2	node20	v3	release.yml ×1
astral-sh/setup-uv	v5	node20	v8	bump.yml ×1, ci.yml ×3, release.yml ×1
dorny/test-reporter	v1	node20	v3	ci.yml ×1

This one needs more care than the other repos. The first four are the same routine bump the sibling Flutter projects already run green. The last two are not:

• astral-sh/setup-uv v5 → v8 crosses three majors. Check the input names still match (caching and version-resolution inputs changed across those releases) before assuming a drop-in swap.
• dorny/test-reporter v1 → v3 crosses two majors and this is the action that publishes check runs, so a silent behaviour change shows up as missing or misattributed test reports rather than a hard failure.

Suggested split: one PR for the four actions/* + action-gh-release bumps (low risk, verifiable by CI), a second for setup-uv and test-reporter after checking their changelogs — so a regression in the risky two does not force a revert of the safe four.