v3.21.0 release tag does not exist on GitHub

The v3.21.0 release on crates.io was published from 4ad1ae6d82608b8e34100f1e215cfbc26aef91db according to its [VCS info file](https://docs.rs/crate/tempfile/3.21.0/source/.cargo_vcs_info.json), but the commit was either not tagged or the tag wasn't pushed to GitHub. I noticed this because Dependabot was unable to provide a diff view for the commits between v3.20.0 and v3.21.0. Ideally Dependabot could figure out the diff from the VCS info file like I did, but for whatever reason it can't. You might also be interested in taking advantage of crates.io's recent support for [trusted publishing](https://crates.io/docs/trusted-publishing). I've got that set up for my crates so that I trigger publishing a release to crates.io by pushing a Git tag to GitHub, which means this situation can't happen.