Upgrade bitflags to 2.0.

[khuey]

[khuey]

Mmm, yes, that's a good point.

[atouchet]

Note that there is a similar existing PR: #400

[hydra]

any chance we can get this or #400 merged? I agree with this comment, https://github.com/image-rs/image-png/pull/400#issuecomment-1913325342

I personally find it easier to deal with a few releases each with small breaking changes that one release which has lots of breaking changes as you can fix them one at a time instead of all in one go. This makes resulting downstream changes/upgrades easier to review for people/teams using this crate.

[fintelia]

Closing in favor of #400