Test fewer Electron versions

[timfish]

Following Chrome releases, Electron releases a new major version every 8 weeks which is 6-7 releases per year which in turn increases our e2e test matrix by 18-21 version/platform combinations per year. Once Electron v21 is released, our test matrix will be 60 combinations.

Our test versions here are populated by this script which keeps the versions updated by PRs that are created by CI.

Electron releases are stable and well tested and we've never seen behaviour change outside of bug fixes on recently released major versions.

• It's important that we test our minimum supported version (v2)
• We should test the last N major versions
  • What is a good value for N?
  • Electron apply bug fixes and security patches to the last 3 major releases
• It would be nice to be able to tag a PR/commit so that ALL versions are tested