Exclude s2n-tls-sys from dependabot

[goatgoose]

Problem:

Dependabot isn't currently running on the rust crates. The list of recent dependabot runs only include github actions. This is likely because #4983 changed the rust crate paths.

However, even before this change, dependabot was unable to run because of the Cargo.toml template file in s2n-tls-sys, which dependabot doesn't understand.

Solution:

To fix dependabot, we should update dependabot.yml to path to the correct rust bindings workspaces. Additionally, we should omit s2n-tls-sys from the scope of dependabot, so it doesn't fail and prevent dependabot checks on the other crates.