Relicenses `zlog`, `ztracing`, and `ztracing_macro` from
GPL-3.0-or-later to Apache-2.0.

GPUI depends on `ztracing`, which depends on `zlog`, so these crates
have to be non-copyleft for GPUI to ship as a permissively licensed
project.

- Switched the `license` field to `Apache-2.0` in all three manifests.
- Added the `LICENSE-APACHE` symlink to `zlog`, and dropped the
`LICENSE-GPL` symlinks from all three. `ztracing` and `ztracing_macro`
already carried both symlinks; now the on-disk license matches the
manifest.
- Declared `license = "Apache-2.0"` for `gpui_util`, which had a
`LICENSE-APACHE` symlink but no `license` field. `script/check-licenses`
doesn't catch that, since it only inspects symlinks.

No source changes; none of these crates carry GPL headers or in-source
license references.

Checked the rest of the subtree while here: the only first-party crates
reachable from `ztracing` via `cargo tree -e normal` are `collections`,
`gpui_util`, `zlog`, and `ztracing_macro`, all Apache-2.0 now.
Third-party deps (`tracing`, `tracing-subscriber`, `chrono`, `log`,
`anyhow`, `tracy-client`) are MIT/Apache.

`script/check-licenses` passes.

Release Notes:

- N/A