ci: Simplify release-plz setup (#65)

Use example configuration from https://release-plz.ieni.dev/docs/github#example-release-pr-and-release (instead of https://blog.orhun.dev/automated-rust-releases/).

Don't need the GitHub PAT token complexity since I'm not using `cargo-dist` here.

.github/workflows/release-plz.yml

@@ -1,4 +1,8 @@
-name: Release automation
+name: Release-plz
+
+permissions:
+  pull-requests: write
+  contents: write
 
 on:
   push:
@@ -15,13 +19,10 @@ jobs:
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
-          token: ${{ secrets.RELEASE_PLZ_TOKEN }}
-
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
-
       - name: Run release-plz
         uses: MarcoIeni/release-plz-action@v0.5
         env:
-          GITHUB_TOKEN: ${{ secrets.RELEASE_PLZ_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}