Set MSRV to 1.72.0

[Rua]

Changelog:

### Public dependency updates
- Rust version: 1.72.0

Rather than speculating about the MSRV forever, I figured it would be best to just set it. I've reverted the ptr::from_ref additions, since those bump it to 1.76.0, which is a bit too new to require IMO. Rust 1.78 is expected to come with a Clippy lint that checks the code against the MSRV, so that should help us realise when we're breaking it. Note that unknown Clippy lints only generate warnings, so we can include them even if they are not supported by the MSRV (as long as CI is using the newest version).

[marc0246]

I like the workspace inheritance. That was long overdue 👍 The readme field can be a inherited as well if you like (it would have to be set to README.md in the root manifest). But it's weird that there's both field.workspace = true and field = { workspace = true }. I would prefer it if you picked one style, doesn't matter which.