Main branch renamed to `trunk`

On 17 June 2022 the main branch of this repository was renamed to `trunk`. For anyone developing with Jetpack locally, you can update your repository by running the following commands:

```
git branch -m master trunk
git fetch origin
git branch -u origin/trunk trunk
git remote set-head origin -a
```

No change is needed on existing PRs as GitHub [will handle this automatically](https://github.com/github/renaming). Thank you!

The issue body documents the repository rename from master to trunk and gives git commands for updating local Jetpack checkouts. Start by checking the repository's contributor or setup documentation for references to the old branch name; done means those references describe trunk and the documented update steps accurately.

Written by the indexing model from the issue text.