ci(api-docs): upgrade to ubuntu 22 and remove conda dependency

Ubuntu 22 has doxygen version 1.9.1 available in apt, which means we
don't need to use conda anymore. This will somewhat simplify the
workflow.

.github/workflows/api-docs.yml

@@ -21,7 +21,7 @@ on:
 
 jobs:
   regen-api-docs:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     permissions:
       contents: write
       pull-requests: write
@@ -35,9 +35,7 @@ jobs:
       - name: Install dependencies
         run: |
           sudo apt-get update
-          sudo env DEBIAN_FRONTEND=noninteractive apt-get install -y python3 luajit
-          conda install -c conda-forge doxygen=1.9.2 msgpack-python
-          echo "$CONDA/bin" >> $GITHUB_PATH
+          sudo env DEBIAN_FRONTEND=noninteractive apt-get install -y doxygen python3 python3-msgpack luajit
 
       - name: Setup git config
         run: |