add automatic labelling to dependabot PRs

[rursprung]

dependabot PRs currently always fail the label check.
there are two options to solve this:

• accept the label dependencies (set by dependabot by default) in the check
• let dependabot set a label which is accepted by the check (see the config docs on how to do this)

as this seems to affect all liquibase repos it'd be good if there's the same solution for them all (though personally i only care about this repo here as i'm not involved in the others).