Clean up features

[Kijewski]

• Use dep: prefix to suppress implicit features: Our msrv is 1.65 by now, so we can safely use this rust 1.60 feature. This is a breaking change, because users could use either features = ["serde-json"] or ["serde_json"] to get the same result. With this change, only the former remains valid.

• The features "mime" and "mime_guess" are deprecated and do nothing since #594. Keeping such deprecated features around can still cause confusion whether one needs to select them or not. It's been 2,5 year since #594 was merged, so I guess we can drop the features.

[Kijewski]

@GuillaumeGomez, do you know if it is possible to generate this page https://docs.rs/crate/askama/0.12.1/features locally?

[GuillaumeGomez]

@GuillaumeGomez, do you know if it is possible to generate this page https://docs.rs/crate/askama/0.12.1/features locally?

We use cargo metadata --format-version 1 and then list the features from there. We could then generate a markdown from that and pass it as argument to rustdoc (like rustdoc features.md, because yes, rustdoc can generate HTML from markdown directly :smile: ).

[djc]

@Kijewski ping?