Update dependencies and keep them up to date

[0xE282B0]

We have updated a lot of dependencies recently and only have a few breaking updates left:

• protobuf v3.2 -> v3.7 (blocked by rust-criu [BUG] Dependency protobuf not correctly specified checkpoint-restore/rust-criu#19)
• rtnetlink v0.13 -> v0.15 (requires code changes)
• syn v1 -> v2 (requires code changes)
• validator v0.16 -> v0.20 (requires code changes)

What do you think about enabling Dependabot to keep our dependencies up to date automatically?
Are there any concerns or alternative approaches we should consider?

[dmah42]

i tried it on another rust project and it broke things, because rust projects don't stick to good semver and they break interfaces all the time.

it's great for notifications though.