Fix e2e tests failure on merge

[NathalieCharbel]

Description

Problem: scheduled/main E2E fails due to runner disk limits

Solution: free disk, manual trigger, concurrency

• Add “Free up disk space” step (remove preinstalled SDKs, prune Docker/caches)
• Replace redundant Docker prune step.
• Enable manual runs via workflow_dispatch.
• Add top-level concurrency to serialise runs per branch and cancel older runs.

Type of Change

• New feature
• Bug fix
• Breaking change
• Documentation update
• Project configuration change

Complexity

Note

Please provide an estimated complexity of this PR of either Low, Medium or High

Complexity:

How Has This Been Tested?

• Unit tests
• E2E tests
• Manual tests

Checklist

The following requirements should have been met (depending on the changes in the branch):

• Documentation has been updated
• Unit tests have been updated
• E2E tests have been updated
• Examples have been updated
• New files have copyright header
• CLA (https://neo4j.com/developer/cla/) has been signed
• CHANGELOG.md updated if appropriate

[stellasia]

Thanks for tackling this issue, it's been pending for a while.