fix: use gh release upload for ZAP report upload to pre-release

[Mysterio-17]

Description

The softprops/action-gh-release action cannot upload files to an already existing release, so the nightly ZAP DAST reports were silently failing to attach to the pre-release. This replaces it with gh release upload --clobber, which properly uploads files to existing releases.

Closes #2333

Changes

• Replaced softprops/action-gh-release with gh release upload "pre-release" --clobber
• Each report file is checked for existence before upload (since ZAP scan uses || true)
• Uses GITHUB_TOKEN for authentication (already available via permissions: contents: write)

[Mysterio-17]

Hello @sydseter , I've replaced softprops/action-gh-release with gh release upload --clobber so ZAP reports are properly uploaded to the existing pre-release.
Kindly have a look at it.