feat: add dependabot config for plugins

[joonseolee]

What this PR does / why we need it:

Adds .github/dependabot.yml template to scaffolded plugins to enable automated dependency updates.

Which issue(s) this PR fixes:

Fixes #2209

Special notes for your reviewer:

• Daily dependency checks for npm, GitHub Actions, and Go modules (when backend is enabled)
• Package grouping to reduce PR noise (Grafana packages and tooling grouped separately)
• Consistent 5 PR limit across all ecosystems

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[joonseolee]

@jackw

I have changed some lines.
Can you check it again?

[jackw]

@joonseolee would you be able to take a look at the last round of feedback? If you don't have the time feel free to let us know and myself or a colleague will address the feedback and get the PR merged. Thanks. ✌️