Bump Docker image to Debian 12 for pure-rust-build

It was previously Debian 10 buster. That version of Debian is past
long term support (LTS), and although third-party Freexian ELTS
updates are publicly available, the image on Docker Hub has not
received any updates since the end of its LTS period. In contrast,
the Debian 11 bullseye and Debian 12 bookworm images continue to be
regularly updated, as of this writing.

Author: EliahKagan

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ on:
 jobs:
   pure-rust-build:
     runs-on: ubuntu-latest
-    container: debian:buster
+    container: debian:bookworm
     steps:
       - uses: actions/checkout@v4
       - name: Prerequisites