CI: dependabot to automate GitHub actions updates

[realeinherjar]

Currently there's a bunch of actions that are outdated, e.g.

bdk/.github/workflows/audit.yml

Line 16 in 2867e88

- uses: actions/checkout@v2

which is already on v4.

Adding a dependabot.yml to update these on a sane interval (weekly?) could help us.
It will automatically create PRs updating the CI GitHub actions.

dependabot was recently implemented in rust-bitcoin, see rust-bitcoin/rust-bitcoin#2052.

[realeinherjar]

Assign it to me, I will add these today after work hours.

[apoelstra]

We did this in rust-bitcoin. I think it's been a net positive. It opens PRs to update versions of various things in the CI pipeline. Usually these are 1-line changes. No visible effects, but keeps us from getting too out of date.