Changes prerequisites headings in tutorials

What does this MR do and why?

This MR updates the "Prerequisites" headings to "Before you begin" in the tutorial pages, as per the changes implemented in this MR: Updates usage of prerequisites in tutorials (!133906 - merged).

In some tutorial pages, the prerequisites appear in the following format used for task topic types, which I have updated to be consistent:

Prerequisites:

- This is a prerequisite.

Related to: https://gitlab.com/gitlab-org/technical-writing/-/issues/887