Combine many dependabot PRs into one

#4135 explored a scripted approach (running client side) of combining multiple PRs into one.

The motivation is that the sheer volume of dependabot PRs creates a weekly multi-hour merge chore, as well as obscuring real PRs by real people - both of which are terrible.

https://github.com/dependabot/dependabot-core/issues/1190 looks like it won't happen any time soon.

https://www.hrvey.com/blog/combine-dependabot-prs looks very promising and I think we should give it a try instead of my previous script attempt. I'd like to verify:

• that, as seen in #4135, we can have the release notes updates automatically. We'd need to:
  • enable merge commits in this repo, and make sure we use them when merging the combined PR
  • (I think) update our release-drafter workflow triggers, so that pull request events trigger release drafter. Even if not required, we should probably do that anyway.
• that merge conflicts are not too big a problem. We'll likely seem them occurring with changes in adjacent lines in build.gradle files, which may or may not be a nuisance.
  • if merge conflicts are a problem, I have written a dirt-simple git merge driver script which seems to work well enough for build.gradle files - perhaps we could explore using this.

Comienza revisando #4135 y el workflow release-drafter al que se hace referencia en el issue. Comprueba si combinar Dependabot PRs con merge commits actualiza correctamente las release notes y, después, evalúa los conflictos de merge en los archivos build.gradle; se considera terminado cuando el workflow es fiable y los conflictos son manejables.