Fix Docker service image references in GitHub Actions workflow

[Copilot]

GitHub Actions workflow was failing because Solr and PostgreSQL service containers used incorrect matrix variables, generating malformed image tags with double colons.

Changes

• Solr service: Use matrix.ckan-solr-version instead of matrix.ckan-version
• PostgreSQL service: Use matrix.ckan-postgres-version instead of matrix.ckan-version

Before/After

# Before: Invalid image reference
services:
  solr:
    image: ckan/ckan-solr:${{ matrix.ckan-version }}
    # Expands to: ckan/ckan-solr:ckan-dev:2.9-py3.9 (invalid: two colons)

# After: Valid image reference  
services:
  solr:
    image: ckan/ckan-solr:${{ matrix.ckan-solr-version }}
    # Expands to: ckan/ckan-solr:2.9-solr8 (valid)

This ensures proper service container initialization across all CKAN versions (2.9, 2.10, 2.11).

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