[Meta] Upgrade GitHub Actions to Node.js 24 compatible versions and add Dependabot

[joseph5wu]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

• Upgrade outdated GitHub Actions to latest versions (Node.js 24 compatible)
• Add .github/dependabot.yml for automatic future upgrade PRs

Motivation

GitHub is deprecating Node.js 20 actions starting June 2, 2026. Several workflows were using older action versions that trigger deprecation warnings.

https://github.com/facebook/fboss/actions/runs/26126542327

Changes

File	Action	Before	After
fboss2-cli.yml	actions/checkout	v4	v6
fboss2-cli.yml	actions/upload-artifact	v4	v6
fboss2-cli.yml	actions/download-artifact	v4	v7
distro_image_build.yml	actions/checkout	v5	v6
github-pages-deploy.yml	actions/upload-pages-artifact	v3	v4

Test Plan

This will trigger the three github action flows and will monitor there's no more warning about outdated versions

[meta-codesync]

@joseph5wu has imported this pull request. If you are a Meta employee, you can view this in D105756484.

[meta-codesync]

@joseph5wu merged this pull request in 52fc2e5.