[Repo Assist] ci: upgrade actions/checkout@v3→v4 and actions/setup-dotnet@v2→v4

🤖 *This is an automated PR from Repo Assist.*

## Summary

Upgrades outdated GitHub Actions versions across all workflow files to address the Node.js 20 deprecation warning. The current CI logs warn:

> actions/checkout@v3, actions/setup-dotnet@v2 are running on Node.js 20 and may not work as expected when Node.js 24 becomes the default (June 2026).

## Changes

| File | From | To |
|------|------|----|
| `build.yml` (×2) | `actions/checkout@v3` | `actions/checkout@v4` |
| `build-and-pack-nightly.yml` | `actions/checkout@v3` | `actions/checkout@v4` |
| `build-and-pack-nightly.yml` | `actions/setup-dotnet@v2` | `actions/setup-dotnet@v4` |
| `bump-fcs.yml` | `actions/checkout@v3` | `actions/checkout@v4` |
| `bump-fcs.yml` | `actions/setup-dotnet@v2` | `actions/setup-dotnet@v4` |
| `release.yml` | `actions/checkout@v3` | `actions/checkout@v4` |

`auto-merge-main-to-nightly.yml` and `copilot-setup-steps.yml` already use `actions/checkout@v4` and were not modified.

## Test Status

These are CI configuration-only changes with no impact on build logic. No code was modified. The existing test matrix continues to run unchanged.

> Generated by [Repo Assist](https://github.com/ionide/FsAutoComplete/actions/runs/23006037651) · [◷](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/bump-fcs.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 23006037651 -n agent-artifacts -D /tmp/agent-artifacts-23006037651

# Create a new branch
git checkout -b repo-assist/eng-update-github-actions-20260312-cf76e4b5ea7bb1ff main

# Apply the patch (--3way handles cross-repo patches)
git am --3way /tmp/agent-artifacts-23006037651/aw-repo-assist-eng-update-github-actions-20260312.patch

# Push the branch and create the pull request
git push origin repo-assist/eng-update-github-actions-20260312-cf76e4b5ea7bb1ff
gh pr create --title '[Repo Assist] ci: upgrade actions/checkout@v3→v4 and actions/setup-dotnet@v2→v4' --base main --head repo-assist/eng-update-github-actions-20260312-cf76e4b5ea7bb1ff --repo ionide/FsAutoComplete
```