workspace: set resolver = 2 in the root Cargo.toml manifest

[cvybhu]

Running cargo check using rust 1.73 prints the following warning message:

warning: some crates are on edition 2021 which defaults to `resolver = "2"`, but virtual workspaces default to `resolver = "1"`
note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest
    Finished dev [unoptimized + debuginfo] target(s) in 0.05s

Documentation about resolver versions can be found here: https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions

Let's use the newer resolver in the whole workspace. The newer resolver is the default that Rust uses since edition 2021.

It also gets rid of the annoying warning.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• [ ] I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.