Change label 'good first patch' to GitHub default 'good first issue'

I propose to change the label good first patch to the GitHub default good first issue for consistency.
Since the label good first issueis a default label, it is most likely the standard label searched by (first time) contributors.

What do you think, @exercism/java ?

If no one objects to this change, the policy for first patches will need an update (wording).