Relax chrono dependency version based on consumer feedback.

[ajrcarey]

Follow-on from #157, #163. Relax the chrono dependency from its current chrono = "0.4, <= 0.4.31" to simply chrono = "0.4" to allow it to float. This will necessarily relax the MSRV of this crate from 1.60 to 1.63 or similar. The aim is to mitigate version conflicts that have been experienced by several consumers since establishing an MSRV of 1.60.

[ajrcarey]

Of course, there is another option: remove the chrono dependency entirely. I believe it's only used in the utils::dates module.

[ajrcarey]

cargo msrv reports that letting the chrono dependency float results in an MSRV of 1.61.

[ajrcarey]

(compared to an MSRV of 1.79 when feature image_025 is enabled)

[ajrcarey]

Updated self-reported MSRV in Cargo.toml. Updated README.

[ajrcarey]

Released as part of crate version 0.8.26.