ci: pin synapse docker version (#41829) - #41839

Conclusion: failure

Steps

• ✅ No merge conflicts
• ❌ QA assured — This PR is missing the 'stat: QA assured' label
• ✅ Mergeable
• ❌ Has milestone or project — This PR is missing the required milestone or project
• ✅ Valid PR title
• ✅ Correct target version