Update Dependabot config to use target-branch for one of the configurations.

[geriux]

This is a follow-up of #6744

Unfortunately, those changes failed with the following error:

Update configs must have a unique combination of 'package-ecosystem', 'directory', and 'target-branch'

After some investigation, I saw there's an issue related to this, where a workaround is suggested and I'm trying out with this PR.

I've added target-branch to the Gutenberg config, which should bypass the error mentioned above.

Since this workaround only works for two configurations, I've merged the Jetpack and Block Experiments since the latter doesn't get too many updates lately so it should only trigger Jetpack updates daily.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[twstokes]

Sounds good @geriux.