Fix mermaid syntax errors in documentation pages

What does this MR do and why?

This MR fixes mermaid "Syntax errors" as described in #441233 (closed).

Two errors are due to the use of <br/> in an unquoted argument to subgraph. Quoting the argument is sufficient to resolve this issue.

See #441248 (closed) how we plan to prevent future failures.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

• https://gitlab.com/gitlab-org/gitlab/-/blob/b8984e9b6636897e7fdee47571be155f17445795/doc/architecture/blueprints/gitlab_services/index.md
• https://gitlab.com/gitlab-org/gitlab/-/blob/922099e03fcdc034a05facb638e2ca01e30a2484/doc/development/gitlab_flavored_markdown/specification_guide/index.md

Before	After

How to set up and validate locally

npm install -g @mermaid-js/mermaid-cli

mmdc -i doc/development/gitlab_flavored_markdown/specification_guide/index.md  -o /tmp/foo.svg

mmdc -i doc/architecture/blueprints/gitlab_services/index.md  -o /tmp/foo.svg

# No failures