Automate routine dependency updates (that dependabot can't)

[legoktm]

We have some dependencies that we need to routinely update, but dependabot can't do them because they're not straightforward package things.

Primarily this includes:

• Rust toolchain updates (e.g. Update Rust toolchain 1.84.1 -> 1.87.0 and rust-init to 1.28.2 securedrop-client#2458)
• securedrop-builder updates (e.g. freedomofpress/securedrop-client@6c0a257)

Infra has some new update tooling that we should be able to leverage for this purpose.