update test matrix + fix shield #9

	# This workflow will build a Java project with Maven
	# For more information see: https://help.github.com/actions/language-and-framework-guides/building-and-testing-java-with-maven

	name: Java CI with Maven

	on:
	push:
	branches: [ master ]
	pull_request:
	branches: [ master ]

	jobs:
	build:

	runs-on: ubuntu-latest
	strategy:
	matrix:
	java: ['1.8','11', '17', '21']

	steps:
	- uses: actions/checkout@v2
	- name: Set up JDK ${{matrix.java}}
	uses: actions/setup-java@v1
	with:
	java-version: ${{matrix.java}}
	- name: Maven -v
	run: mvn -v
	- name: Cache Maven packages
	uses: actions/cache@v2
	with:
	path: ~/.m2
	key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
	restore-keys: ${{ runner.os }}-m2
	- name: Build with Maven
	run: mvn -B package --file pom.xml
	# don't work at the moment it seems, see https://github.com/trautonen/coveralls-maven-plugin/issues/136
	# - name: Coverage
	# run: mvn jacoco:report coveralls:report

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