Upgrade nokogiri (development dependency) to 1.13.x

[ggrossetie]

We might be able to remove the BUNDLE_BUILD__NOKOGIRI=--use-system-libraries environment variable in:

asciidoctor/.github/workflows/ci.yml

Line 76 in cd9b07c

run: echo 'BUNDLE_BUILD__NOKOGIRI=--use-system-libraries' >> $GITHUB_ENV

I believe we are using --use-system-libraries because of this issue: flavorjones/mini_portile#101
Since this issue is now fixed in version 2.5.2 and Nokogiri 1.13.x is using version 2.8+ of mini_portile, I think it would make sense to upgrade.

Initial discussion: https://asciidoctor.zulipchat.com/#narrow/stream/279656-dev-.F0.9F.9B.A0.EF.B8.8F/topic/bundle.20install.20in.20asciidoctor.20with.20Ruby.203.2E1.20.28on.20Ubuntu.29

[mojavelinux]

The main reason we're using --use-system-libraries is because it's takes an excruciatingly long time to rebuild nokogiri. By linking instead, it speeds up the installation dramatically.

[mojavelinux]

The problem with upgrading is that it requires Ruby >= 2.6 and we test back to Ruby 2.3.

We are reevaluating how we drop old versions of Ruby, but suffice to say, core has to be prudent with the minimum Ruby version to support all the Linux distributions and macOS. So I don't think we can make this jump yet.

What exactly is the reason for wanting to upgrade? All the tests are passing in CI right now.

[ggrossetie]

As an intermediate step we could upgrade to Nokogiri 1.12 which uses mini_portile 2.6.1 and requires Ruby 2.5.

The main reason is that it does not install on Ubuntu 20.4 LTS with a recent version of Ruby when building from source (i.e. without --use-system-libraries).
It's not a big deal but we might want to add a note in the contributing guide.

[mojavelinux]

This was resolved by #4339.