chore(ci): drop invalid upload-artifact input from gh-release action

softprops/action-gh-release@v3 was logging a warning on every
release run because the workflow passed `upload-artifact: true`,
which isn't in the action's input schema (the action discovers
files via the `files:` glob list — `upload-artifact` is residue
from an earlier action version). The release succeeded anyway,
but the annotation showed up in every run summary.

Removed the unrecognised key. No behavioural change — the wheel,
sdist, and SBOM still get attached via the `files:` block.

Author: imanimanyara

.github/workflows/release.yml

@@ -98,4 +98,3 @@ jobs:
             dist/*.whl
             dist/*.tar.gz
             dist/shimkit-sbom.spdx.json
-          upload-artifact: true