Bump msrv to 1.70

[Lorak-mmk]

Checking MSRV locally is frustrating, because it requires switching toolchain and renaming files, but it is even more frustrating because updating cargo index before 1.70 is so sloooooow.
1.70 switched to sparse cargo index protocol by default, which makes checking the crate much faster. Let's bump MSRV to improve developer experience.

I also added "rust-version" field to Cargo.toml files, because this is the official place to specify MSRV and in the future will be used by Cargo during dependency resolving.

1.70 is over a year old (released on 1st June 2023), so I doubt this bump will cause any problems.
Related: we will need to decide on MSRV policy past 1.0. I propose to settle on 6 month policy (= our MSRV must always be at least 6 months old).
Rust ecosystem seems to not be very conservative on this:

• Tokio has the exact same MSRV policy that I propose here: https://github.com/tokio-rs/tokio
• time crate guarantess support of current stable and 2 previous versions (= minimum 4 months )
• chrono is a bit more conservative, with 1.61 MSRV: https://docs.rs/chrono/latest/chrono/

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.

[github-actions]

cargo semver-checks found no API-breaking changes in this PR! 🎉🥳
Checked commit: 1da6c85

[muzarski]

I propose to settle on 6 month policy (= we only support Rust versions released during last 6 months)

Then we will not support versions that were released more than 6 months ago. You probably meant that MSRV should be a version that is at least 6 months old (AFAIU, this is the tokio's policy), correct? If so, then I agree that such policy makes sense.

[Lorak-mmk]

I propose to settle on 6 month policy (= we only support Rust versions released during last 6 months)

Then we will not support versions that were released more than 6 months ago. You probably meant that MSRV should be a version that is at least 6 months old (AFAIU, this is the tokio's policy), correct? If so, then I agree that such policy makes sense.

Yes, you're right, and that is what I'd like to propose.

And just like Tokio, this wouldn't be automatic: if we don't see the benefit in bumping MSRV we can stay on older one for much longer.

[Lorak-mmk]

@muzarski I updated the description regarding the 6 months policy.