Run Dependabot independently on each branch

[hugovk]

Rather than backporting Dependabot updates, which will pretty much always have conflicts due to different workflows, and which we often forget to do (causing more conflicts), let's have Dependabot run on each branch.

The config belongs in main, and unfortunately needs repeating, but each block is fairly small.

• https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference#target-branch-
• https://docs.github.com/en/code-security/tutorials/secure-your-dependencies/customizing-dependabot-prs#targeting-pull-requests-against-a-non-default-branch

(Renovate would allow us to use a regex and avoid the repetition, but that's a bigger move involving installing a new app. But I'm a happy Renovate user in other projects, so it's always an option for later.)

[ezio-melotti]

It's unfortunate that there's no easy way to do it (unless target-branch: "3.*" works, but it's not documented).

This feature has been requested upstream before:

• dependabot/dependabot-core#2159

Here is the relevant documentation: https://docs.github.com/en/code-security/tutorials/secure-your-dependencies/customizing-dependabot-prs#targeting-pull-requests-against-a-non-default-branch

Also note this:

Dependabot raises pull requests for security updates against the default branch only. If you use target-branch, then as a result, all configuration settings for that package manager will then only apply to version updates, and not security updates.

[ezio-melotti]

I would move this to the top, so that all the main sections are together. Perhaps it should also be duplicated for all branches.

[ezio-melotti]

Since we are duplicating all lines and readability is less of a concern, maybe we could use this compact form instead:

Suggested change

	labels:
	- "skip issue"
	- "skip news"
	groups:
	actions:
	patterns:
	- "*"
	labels: ["skip issue", "skip news"]
	groups:
	actions:
	patterns: ["*"]

These could be further reduced to this if we wanted to:

Suggested change

	labels:
	- "skip issue"
	- "skip news"
	groups:
	actions:
	patterns:
	- "*"
	labels: ["skip issue", "skip news"]
	groups: { actions: { patterns: ["*"] } }

If we applied the latter to the whole block, each block will only take 7 lines (instead of 14)