Only allow "Squash and Merge" PR merge strategy

[andrii-i]

Problem

While based on the repo history "Squash and Merge" is a preferred PR merge strategy, all PR merge strategies are currently allowed resulting in need to explicitly choose "Squash and Merge", "Merge commit" strategy being the default one proposed in UI.

Proposed Solution

Only allow "Squash and Merge" PR merge strategy. This can be done in "Settings > General > Pull Requests". This will result in "Squash and Merge" being only available and default merge strategy which will ensure clean commit history and improve merge / PR user experience.

If everyone are OK with this, I can enable this myself.

[brichet]

Thanks @andrii-i , it looks good to me 👍.

[andrii-i]

Done, only "Squash and merge" is now allowed 🎉