v0.6.0

fix(release-ci): replace retired GitHub-hosted runner images

The "Build precompiled NIFs" workflow pinned runner images that GitHub
has since retired: ubuntu-20.04, windows-2019, and macos-13. Jobs that
request a nonexistent runner image never get assigned a runner — every
matrix job sits in `queued` and is cancelled at the 24h ceiling, so no
precompiled NIFs (and therefore no Hex release) can be produced. The
Tests workflow is unaffected because it uses `ubuntu-latest`.

Move to currently supported, non-deprecated images:
- ubuntu-20.04  -> ubuntu-22.04
- windows-2019  -> windows-2022
- macos-13      -> macos-15

These favour the oldest images that are still supported (not the newest)
for portability, but avoid ones already flagged for retirement:
macos-14 is currently marked deprecated, so macos-15 is used instead.

macos-15 is Apple Silicon; both darwin targets are already built with an
explicit `--target`, so x86_64-apple-darwin cross-compiles fine there.

Also build x86_64-unknown-linux-gnu via `cross` (use-cross: true). It was
the only target built natively on the host, so bumping the host to
ubuntu-22.04 would have raised its glibc floor from 2.31 to 2.35 and cut
portability of the precompiled artifact. Building it in the cross image
keeps the glibc floor low and host-independent, matching the intent of
shipping broadly compatible precompiled NIFs.