gst-plugin-version-helper: cargo intentionally provides bogus mtime on files so can't use it as release date

See https://github.com/rust-lang/cargo/issues/10285

For releases we'll have to provide another mechanism than looking at the Cargo.toml's mtime. The most obvious approach would be to add a release date directly to Cargo.toml as a custom

[package.metadata.gstreamer]
release_date = "2023-07-20"

This would then have to be always updated in sync together with the version numbers.