update GHA mdbook version to the one in the lockfile

Author: lqd

.github/workflows/mdbook.yml

@@ -33,7 +33,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     env:
-      MDBOOK_VERSION: 0.4.40
+      MDBOOK_VERSION: 0.4.43
     steps:
       - uses: actions/checkout@v4
       - name: Install mdBook