Test legacy rubies on Ubuntu 20.04

Attempt to prevent segmentation faults

.github/workflows/legacy_ruby.yml

@@ -12,13 +12,12 @@ permissions:
 jobs:
   test:
     name: Legacy Ruby specs
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.ruby < '2.3' && 'ubuntu-20.04' || 'ubuntu-latest' }}
 
     strategy:
       matrix:
         ruby-version: ['2.1', '2.2', '2.3', '2.4', '2.5']
         gemfile: [am_3.2, am_4.0, am_4.1, am_4.2, am_5.0, am_5.1, am_5.2, am_6.0, am_6.1]
-        channel: [stable]
 
         exclude:
           - ruby-version: '2.1'
@@ -70,11 +69,6 @@ jobs:
       BUNDLE_GEMFILE: gemfiles/${{ matrix.gemfile }}.gemfile
       TEST_CONFIG: ./spec/config.github.yml
 
-    # Ruby < 2.3 fail with segmentation fault so they are allowed to fail
-    # On ubuntu-20.04, also 2.3 fails, maybe this is an issue with libcurl
-    # (should investigate)
-    continue-on-error: ${{ matrix.channel != 'stable' || matrix.ruby < '2.3' }}
-
     steps:
     - uses: actions/checkout@v4
     - name: Set up Ruby