Fix release CI: upgrade mlugg/setup-zig to v2 and cut 0.0.3

`mlugg/setup-zig@v1` is deprecated and only knows the pre-0.14
tarball name (`zig-linux-x86_64-<ver>.tar.xz`), so every mirror —
and the official ziglang.org/builds — returned 404 for Zig 0.15.2
on both the v0.0.1 and v0.0.2 release runs. v2 uses the new
`zig-x86_64-linux-<ver>.tar.xz` layout that Zig switched to in
0.14+.

Also rolls the existing CHANGELOG `[Unreleased]` work into a
dated `[0.0.3]` section and adds the CI fix to its Fixed list.

.gitea/workflows/release.yml

@@ -16,7 +16,7 @@ jobs:
           go-version-file: go.mod
           cache: true
 
-      - uses: mlugg/setup-zig@v1
+      - uses: mlugg/setup-zig@v2
         with:
           version: 0.15.2