chore: Remove rust-major dependabot group

[samuelburnham]

Dependabot has difficulty grouping Rust major version dependency updates due to non-trivial Cargo.lock changes, so it falls back to opening one PR per major version (see #406, #405). This PR removes the rust-major group altogether since it will likely never be used.

Other changes:

• Removes PRs for indirect dependencies to reduce noise, since we have Dependabot alerts and cargo-deny to notify for security updates. Otherwise a periodic cargo update will take care of them.
• Raises the open PR limit to 10 so we are less likely to miss updates for major versions.
• Adds a one week cooldown to mitigate supply chain attacks.