.github: Add dependabot config and thus enable dependabot

[mxinden]

Just like paritytech/substrate#7509 I am suggesting to enable Dependabot for the Polkadot repository.

With this pull request I suggest to enable Dependabot for this repository. Dependabot will scan the dependencies used in Substrate (both Cargo.toml and Cargo.lock) and create pull requests whenever newer versions of those dependencies are available.

We are using Dependabot in rust-libp2p since about 2 months now. While annoying at first, I think it is well worth it, reducing the long-term toil keeping dependencies up-to-date as well as staying in sync with recent (security) bug fixes.

Please don't just merge here but signal your consent / dissent for now. Keeping dependencies up-to-date is a lot of work and thus a team effort.

[ordian]

I think this is a good idea in general to keep the dependencies up to date. It also helps to identify problems with a dependency update early on.

[bkchr]

Same comment as on substrate.

[mxinden]

Large dependency update for Polkadot happened in #1946.

[mxinden]

bot merge

[ghost]

Trying merge.