Update more deprecated github actions

[boxydog]

https://github.com/tidyverts/fable/actions/runs/7721580515 shows that actions/checkout@v2 and r-lib/actions/setup-pandoc@v1 are deprecated.

This updates them.

[mitchelloharawild]

Thanks. I've updated these to work better for this package's needs.

[boxydog]

FYI if you add something like this as dependabot.yml in your .github folder, it will check for out of date github actions:

# See https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/enabling-and-disabling-version-updates
# and https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

version: 2
updates:
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "monthly"

I suspect it will find some.

[mitchelloharawild]

Appreciate the suggestion, I'll keep that in mind!