[Repo Assist] ci: update GitHub Actions to latest versions (checkout@v4, setup-dotnet@v4)

🤖 *This is an automated pull request from Repo Assist.*

## Problem

Several GitHub Actions workflow files were using outdated action versions and a deprecated workflow command:

| File | Issue |
|------|-------|
| `build.yml` (build job) | `actions/checkout@v3` |
| `build.yml` (analyze job) | `actions/checkout@v3` |
| `build-and-pack-nightly.yml` | `actions/checkout@v3`, `actions/setup-dotnet@v2` |
| `build-and-pack-nightly.yml` | `::set-output` deprecated command |
| `release.yml` | `actions/checkout@v3` |

`actions/checkout@v4` has been available since September 2023, and `actions/setup-dotnet@v4` since early 2024. The `::set-output` workflow command was deprecated in GitHub Actions in October 2022 and replaced by writing to `$GITHUB_OUTPUT`.

## Changes

- `actions/checkout@v3` → `@v4` in all three workflow files
- `actions/setup-dotnet@v2` → `@v4` in `build-and-pack-nightly.yml`
- `echo "::set-output name=date::..."` → `echo "date=..." >> $GITHUB_OUTPUT` in `build-and-pack-nightly.yml`

## Benefits

- Resolves Node.js 16 deprecation warnings from checkout@v3 (GitHub Actions now requires Node.js 20+)
- Eliminates deprecated `::set-output` warnings that appear in nightly build logs
- Stays current with upstream security patches in these actions

## Test Status

Workflow-only change — no source code or tests modified. No local build/test run needed.
These changes are safe; checkout@v4 and setup-dotnet@v4 are drop-in replacements with the same interface.

> Generated by [Repo Assist](https://github.com/ionide/FsAutoComplete/actions/runs/23198566450) · [◷](https://github.com/search?q=repo%3Aionide%2FFsAutoComplete+%22gh-aw-workflow-id%3A+repo-assist%22&type=pullrequests)
>
> To install this [agentic workflow](https://github.com/githubnext/agentics/tree/346204513ecfa08b81566450d7d599556807389f/workflows/repo-assist.md), run
> ```
> gh aw add githubnext/agentics/workflows/repo-assist.md@346204513ecfa08b81566450d7d599556807389f
> ```

---

> [!WARNING]
> 🛡️ **Protected Files — Push Permission Denied**
>
> This was originally intended as a pull request, but the patch modifies protected files: `.github/workflows/build-and-pack-nightly.yml`, `.github/workflows/build.yml`, `.github/workflows/release.yml`.
>
> The push was rejected because GitHub Actions does not have `workflows` permission to push these changes, and is never allowed to make such changes, or other authorization being used does not have this permission. A human must create the pull request manually.

To create a pull request with the changes:

```sh
# Download the patch from the workflow run
gh run download 23198566450 -n agent-artifacts -D /tmp/agent-artifacts-23198566450

# Create a new branch
git checkout -b repo-assist/eng-update-github-actions-2026-03-17-90a5c06e08ef963f main

# Apply the patch (--3way handles cross-repo patches)
git am --3way /tmp/agent-artifacts-23198566450/aw-repo-assist-eng-update-github-actions-2026-03-17.patch

# Push the branch and create the pull request
git push origin repo-assist/eng-update-github-actions-2026-03-17-90a5c06e08ef963f
gh pr create --title '[Repo Assist] ci: update GitHub Actions to latest versions (checkout@v4, setup-dotnet@v4)' --base main --head repo-assist/eng-update-github-actions-2026-03-17-90a5c06e08ef963f --repo ionide/FsAutoComplete
```