Change Gradle commands to use the Gradle Wrapper

[yeikel]

Example:

name: Java CI

on: [push]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Build with gradle
        run: gradle test

Unix runners:

name: Java CI

on: [push]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Build with gradle
        run: ./gradlew test

Windows Runners:

 name: Java CI

on: [push]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Build with gradle
        run: gradlew test

Same as #57 but with the Gradle Wrapper

[timtebeek]

Hope you don't mind I unassigned you @yeikel
I want to open this issue back up again to anyone to pick up.
Feel free to open up a draft pull request if you'd like to contribute!