Bump CI to Ubuntu 20.04 + new checkout action

[abejgonzalez]

Ubuntu 18.04 is being deprecated in December so let's bump it in the workflows. Additionally, bump the checkout action since it is also giving deprecated warnings.

Related PRs / Issues

UI / API Impact

Verilog / AGFI Compatibility

Contributor Checklist

• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you add Scaladoc/docstring/doxygen to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous prints/debugging code?
• Did you state the UI / API impact?
• Did you specify the Verilog / AGFI compatibility impact?

• If applicable, did you regenerate and publicly share default AGFIs?

• If applicable, did you apply the ci:fpga-deploy label?

• If applicable, did you apply the Please Backport label?

Reviewer Checklist (only modified by reviewer)

Note: to run CI on PRs from forks, comment @Mergifyio copy main and manage the change from the new PR.

• Is the title suitable for inclusion in the changelog and does the PR have a changelog:<topic> label?
• Did you mark the proper release milestone?
• Did you check whether all relevant Contributor checkboxes have been checked?

[abejgonzalez]

@Mergifyio rebase main

[mergify]

rebase main

☑️ Nothing to do

• any of:
  • #commits-behind>0 [:pushpin: rebase requirement]
  • -linear-history [:pushpin: rebase requirement]
• -closed [:pushpin: rebase requirement]

[abejgonzalez]

@Mergifyio rebase main

[mergify]

rebase main

❌ Base branch update has failed

Git reported the following error:

Rebasing (1/2)
Rebasing (2/2)
error: could not apply 66aab427... Bump CI checkout version
Resolve all conflicts manually, mark them as resolved with
"git add/rm <conflicted_files>", then run "git rebase --continue".
You can instead skip this commit: run "git rebase --skip".
To abort and get back to the state before "git rebase", run "git rebase --abort".
Could not apply 66aab427... Bump CI checkout version
Auto-merging .github/workflows/firesim-run-tests.yml
CONFLICT (content): Merge conflict in .github/workflows/firesim-run-tests.yml

err-code: DA1FA