DEV: allow non-squash merges

[tylerjereddy]

Looks like the repo doesn't allow individual commits in a PR branch to be preserved on merging (see below). The debate around this stuff isn't super interesting, but I'd argue that leaving the decision about whether individual commits in a branch merit preservation is best left to the reviewer/merger, rather than having a hard-block at the repo level. Sometimes individual commits are complex and have well-written commit messages, and could be productively revisited later on, etc.

Also, pykokkos repo doesn't have this restriction, FWIW.