Add dependabot for 3.x branch

[slawekjaranowski]

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[cdouillard]

Hi @slawekjaranowski,

Since this PR was merged, I noticed that there were no PRs opened by dependabot on the main branch (dependency PRs were only open on the maven-compiler-plugin-3.x branch).

Yet there is, for example, a newer version available for mockito-core (5.15.2).

Maybe it's because of this PR: #174. Should it be reopened ?

[slawekjaranowski]

Ok #174 reopened