Use GitHub Alerts for Deprecated Dependency warnings

[HonkingGoose]

Describe the proposed change(s).

Blocked by:

• Re-enable deprecated dependency warnings #5098

Use GitHub's Alert styling to make the warning stand out more. Of course this will only work on GitHub. 😉 So something like this:

Warning

Renovate found the following deprecated dependencies:

• Deprecated-stuff-1
• Deprecated-stuff-2

You should review the listed dependencies, and either request a update from Renovate via this dashboard, or upgrade manually.

Here's the Markdown for the mock-up above:

> [!WARNING]
> Renovate found the following deprecated dependencies:
> 
> - Deprecated-stuff-1
> - Deprecated-stuff-2
>
> You should review the listed dependencies, and either request a update from Renovate via this dashboard, or upgrade manually.

Read the GitHub Docs, Basic writing and formatting syntax, Alerts for more information.

If [!WARNING] is too eye-catching, we can select a different alert level.

[rarkins]

I'm already doing this as part of my PR #29694

FYI we can put this in source code which is valid on all platforms:

> ⚠ **Warning**\n> \n

And then our github platform massageMarkdown code massages it to this:

> [!WARNING]