feat(CSS): define default styles for <code> element

What does this MR do and why?

The <code> and <pre> HTML elements have default styles applied from Bootstrap. These styles are inconsistent with GitLab implementations so GitLab UI consumers are overriding with very similar styles:

• GitLab: https://gitlab.com/gitlab-org/gitlab/-/blob/1e99a32c3a526b91fa59cb509abde7f78fb7ed54/app/assets/stylesheets/bootstrap_migration_reset.scss#L61-69
• design.gitlab.com: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/blob/46ddbf25b48b911332a7ba9b8531d3fa29cff39a/assets/stylesheets/framework/_typography.scss#L57-68

This MR proposes shifting-left on <code> and <pre> styles to be applied in GitLab UI, remove Bootstrap styles, and eventually remove styles from consumers so that they are visually consistent.

Note: !5002 (merged) adds basic typography stories to validate changes to <code> and <pre> elements in https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/scss-typography--default

References

• Integration branch gitlab!191101 (closed)

How to set up and validate locally

1. View typography story in Storybook
2. Validate <code> and <pre> visual rendering

MR acceptance checklist

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