Update readme, release notes, metadata for 0.23.13

[197g]

It's been a while since the last release but we have some major new features—notably AVIF support—as well as fixes.

[197g]

#1417 (comment) On hold until we've figured out if there is a hole in our test coverage from the recent CI rework.

[fseegraeber]

The --no-default-features seems to override single --features for cargo test.

The println! here is also kind of misleading:

image/tests/reference_images.rs

Lines 153 to 156 in 6d55953

	#[test]
	fn check_references() {
	process_images(REFERENCE_DIR, Some("png"), |base, path, decoder| {
	println!("check_references {}", path.display());

since if you run with e.g. cargo test --no-default-features --features gif -- --nocapture it will print the path, but the actual test will not be executed:

image/tests/reference_images.rs

Lines 193 to 194 in 6d55953

	#[cfg(feature = "gif")]
	if format == Some(image::ImageFormat::Gif) {

If I run all tests locally on the latest master there are no failures though.