Migrate to a less uptight Dependabot config

I'll likely want to migrate to https://github.com/python/cpython/blob/main/.github/dependabot.yml in order to reduce noise and simplify things. We don't really need Dependabot to mess with other branches as Patchback would include version bumps in backports. And getting every patch version update every day is rather excessive, unless it's a security release.

Originally posted by @webknjaz in https://github.com/aio-libs/aiohttp/pull/13727#pullrequestreview-5211476191

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@aiolibsbot implement and use no-indent sequences, with leading start+end YAML markers.