Darken blockquote text in Markdown styles

What does this MR do and why?

Problem: Blockquotes in issue and MR comments are rendered in a grey-ish colour, which lacks contrast. As a result, the text can be difficult to read.

Solution: This MR darkens the blockquote text based on the Typescale specifications found in gitlab-org/gitlab-services/design.gitlab.com#1062 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Create a blockquote in any place where you can use Markdown like issues, MRs, Wikis, etc.
• You will notice that the blockquote styles are darker.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #231405 (closed)