chore: Update workflow actions to Node 24 runtime + watch github-actions (#2486)

## Summary

Audit of CI/CD workflows (`.github/workflows/**`, `azure-pipelines.yml`) for outdated actions, focused on the Node 20 → Node 24 runner deprecation. Most actions were already migrated; this PR cleans up the remaining stragglers and closes the gap that let them drift.

## Changes

| Action | File(s) | From | To | Reason |
|---|---|---|---|---|
| `softprops/action-gh-release` | `create-release.yml`, `build-tool-release.yml` | `v2` | `v3` | v2 still runs Node 20; v3 is a pure Node 24 runtime move, inputs unchanged (drop-in) |
| `dotnet/nbgv` | `create-release.yml` | `v0.5.1` | `v0.5.2` | Node 24 runtime bump |
| `SethCohen/github-releases-to-discord` | `post-release-discord.yml` | `v1.19.0` | `v1.20.0` | Latest; adds manual-dispatch test support |
| Dependabot | `dependabot.yml` | nuget only | + `github-actions` (weekly) | Auto-PR future action bumps instead of manual audits |

## Already current (no change)

`actions/checkout@v6`, `actions/setup-dotnet@v5`, `actions/upload-artifact@v7`, `actions/download-artifact@v8`, and `signpath/...@v2` are all on current majors running the Node 24 runtime. The Azure tasks (`UseDotNet@2`, `NuGetAuthenticate@1`) are current as well.

## Notes

- All target versions verified against GitHub's release API.
- `action-gh-release@v3` release notes confirm it's a runtime-only change with no input/behavior changes — safe drop-in for both usages.

.github/dependabot.yml

@@ -9,3 +9,7 @@ updates:
     directory: "/" # Location of package manifests
     schedule:
       interval: "daily"
+  - package-ecosystem: "github-actions"
+    directory: "/" # Watches .github/workflows/**
+    schedule:
+      interval: "weekly"