Upgrade jekyll to 3.9.0

[brad]

The current version of Jekyll has security vulnerabilities reported by Snyk.io. This PR fixes them with an upgrade to Jekyll

Before

After

I rebuilt the docs to make sure it didn't break anything and it builds fine, except I couldn't build until I added kramdown-parser-gfm to the Gemfile

[alexandrnikitin]

That's great! Thank you so much! We wanted to update the docs dependencies for a long time.

What ruby version did you use? We use 2.5 but maybe it's worth to update it too.

NSubstitute/.github/workflows/build_and_test.yml

Lines 50 to 54 in f470b5d

	- name: Setup Ruby
	uses: ruby/setup-ruby@v1
	with:
	ruby-version: '2.5'
	bundler-cache: true

[brad]

@alexandrnikitin No problem! I used Ruby 2.7.7, I've made the update to build_and_test.yml. I hope that will do it