Remove ubuntu 16.04 from self-tests, out of support

And being removed from the GHA setup-php

https://github.com/shivammathur/setup-php
shivammathur/setup-php#452

So, jumping to 20.04 and also adding some mising php80 runs.

.github/workflows/test.yml

@@ -25,7 +25,7 @@ jobs:
   citest:
     name: CI test
     needs: selftest
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-20.04
 
     services:
       postgres:
@@ -42,9 +42,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - php: '7.4'
+          - php: '8.0'
             moodle-branch: 'master'
-          - php: '7.4'
+          - php: '7.3'
+            moodle-branch: 'master'
+          - php: '8.0'
             moodle-branch: 'MOODLE_311_STABLE'
           - php: '7.4'
             moodle-branch: 'MOODLE_310_STABLE'
@@ -65,7 +67,7 @@ jobs:
       uses: shivammathur/setup-php@v2
       with:
         php-version: ${{ matrix.php }}
-        extensions: pgsql, zip, gd, xmlrpc, soap
+        ini-values: max_input_vars=5000
         coverage: none
 
     - name: Initialise moodle-plugin-ci