Proposal: Enable branch update suggestions

[bryevdv]

@bokeh/core I'd like to propose turning this option on:

In case you push a PR and happen to be behind the base branch, the GH web UI will add a button to update to latest (if possible) right there on the PR page. This seems to be new, I've encountered this in another project and it was convenient.

Let me know if there are any concerns, if I don't hear anything I will plan to turn it on next week.

[mattpap]

As long as it's only a suggestion, it's perfectly fine to enable.

[bryevdv]

Yes it is a button you can push or not push.

[bryevdv]

Here is what it looks like in practice:

I have mixed feelings about merge commit as the default, and I don't think it can be changed at present. But given we squash merge PRs I don't actually think it would matter much in practice.