New Pull Request template refers to a `dev` branch, which does not exist

[yifanmai]

The New Pull Request template says:

**BEFORE YOU START**: please make sure your pull request is against the `dev` branch.
We cannot accept pull requests against the `main` branch.

We should either change this, or create the dev branch.

[dlwh]

Yeah, the dev branch lived for a long long time and we deleted it not that long ago. I sort of prefer doing work off of main, but open to going back to the way it was. @J38 @yifanmai what do y'all think?

[yifanmai]

I prefer working off main and avoiding the extra work of periodically syncing the branches.