Fix the behavior of the GitLab AppSec SAST comment

What does this MR do and why?

Fix the behavior of the GitLab AppSec SAST comment. Resolves Fix SAST bot comments (gitlab-com/gl-security/product-security/appsec/sast-custom-rules#39 - closed).

Previously, the comment would have the warning followed immediately by the footer comment. This was due to an insufficient number of newlines, based on how GitLab Flavored Markdown works.

The comment would also fail to apply a quick action. This was due to it being prefixed by two spaces.

Example of bug

From !154910 (comment 1983774478)

Bug explanation

Newlines

A line break is inserted (a new paragraph starts) if the previous text is ended with two newlines, like when you press Enter twice in a row. If you only use one newline (press Enter once), the next sentence remains part of the same paragraph. Use this approach if you want to keep long lines from wrapping, and keep them editable:

• Bug: SAST warning and the footer text run together in the same paragraph, due to insufficient newlines
• Fix: go from one \n to two two \n\ns

Quick actions not applied

Heredoc syntax:

You may indent the ending identifier if you place a “-” after <<:

...

To have indented content as well as an indented closing identifier, you can use a “squiggly” heredoc, which uses a “~” instead of a “-” after <<:

https://ruby-doc.org/3.2.1/syntax/literals_rdoc.html#label-Here+Document+Literals

• Bug: Quick action not applied due to it being prefixed with whitespace ( /label ...)
• Fix: Use ~ syntax to remove indenting spaces

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