actions: move tests from Ubuntu 18 to 20

These are the tests that try to make sure old library version really
still work as expected. Increases PHP to 7.3 because 7.2 has errors.

.github/actions/build-nominatim/action.yml

@@ -1,10 +1,10 @@
 name: 'Build Nominatim'
 
 inputs:
-    ubuntu:
+    flavour:
         description: 'Version of Ubuntu to install on'
         required: false
-        default: '20'
+        default: 'ubuntu-20'
     cmake-args:
         description: 'Additional options to hand to cmake'
         required: false
@@ -23,10 +23,10 @@ runs:
             sudo rm -rf /opt/hostedtoolcache/go /opt/hostedtoolcache/CodeQL /usr/lib/jvm /usr/local/share/chromium /usr/local/lib/android
             df -h
           shell: bash
-        - name: Install prerequisites
+        - name: Install${{ matrix.flavour }} prerequisites
           run: |
             sudo apt-get install -y -qq libboost-system-dev libboost-filesystem-dev libexpat1-dev zlib1g-dev libbz2-dev libpq-dev libproj-dev libicu-dev liblua${LUA_VERSION}-dev lua${LUA_VERSION}
-            if [ "x$UBUNTUVER" == "x18" ]; then
+            if [ "$FLAVOUR" == "oldstuff" ]; then
                 pip3 install MarkupSafe==2.0.1 python-dotenv psycopg2==2.7.7 jinja2==2.8 psutil==5.4.2 pyicu==2.9 osmium PyYAML==5.1 sqlalchemy==1.4 datrie asyncpg
             else
                 sudo apt-get install -y -qq python3-icu python3-datrie python3-pyosmium python3-jinja2 python3-psutil python3-psycopg2 python3-dotenv python3-yaml python3-asyncpg
@@ -34,7 +34,7 @@ runs:
             fi
           shell: bash
           env:
-            UBUNTUVER: ${{ inputs.ubuntu }}
+            FLAVOUR: ${{ inputs.flavour }}
             CMAKE_ARGS: ${{ inputs.cmake-args }}
             LUA_VERSION: ${{ inputs.lua }}