Introduce Mermaid 12 behind the use_mermaid_v12 FF

What does this MR do and why?

Upgrades to Mermaid 12 behind an FF, the same way we did the 10 to 11 major version upgrade. We configure some settings whose defaults changed so that diagrams don't change in proportion.

pipeline:skip-router-sync added because the routes have been added, but the job is complaining about other routes getting added/removed, which this MR isn't touching.

References

• Use Mermaid 12 (#631203)
• [FF] `use_mermaid_v12` -- Use Mermaid 12 instea... (#631204)

Screenshots or screen recordings

dotcom	FF off	FF on

Source used:

X:

```mermaid
flowchart LR
  A[Start] --> B{Review}
  B -->|Yes| C[Done]
  B -->|No| A
```

Y:

```mermaid
usecase-beta
  actor User
  User --> Login(Sign in)
```

How to set up and validate locally

1. Create a work item with the above source.
2. It should look like the "Before" image.
3. Check out this branch.
4. Refresh the page; it should still look like the "Before" image. (Don't skip this step! This is what verifies the FF off state matches what we have on master right now, which isn't a given due to the complexity of permitting either Mermaid version to load.)
5. Enable the use_mermaid_v12 FF for the project or group you created the work item in.
6. Refresh the page. It should look like the "After" image.

MR acceptance checklist

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