build: bump version number to 13.6.0 (#1092) #2333

Job	Run time
java_matrix	4s
build (macos-latest)	3m 29s
download_test_data	6s
build (ubuntu-latest)	3m 30s
unit_tests (21)	3m 22s
unit_tests (25)	3m 26s
spotbugs	6m 56s
dependency_analysis	1m 0s
test_run-groovy	33s
test_coatjava (ubuntu-latest, kpp)	3m 5s
test_coatjava (ubuntu-latest, eb-epc)	2m 22s
test_coatjava (ubuntu-latest, eb-enc)	2m 40s
test_coatjava (ubuntu-latest, eb-eg)	2m 15s
test_decoder	3m 7s
test_clara	3m 37s
test_coatjava (macos-latest, eb-ep, ./run-eb-tests.sh -100 electronproton)	3m 55s
test_coatjava (ubuntu-latest, eb-ep)	2m 7s
test_coatjava (ubuntu-latest, eb-eftpi)	4m 39s
release_build	0s
generate_documentation	1m 2s
deploy_web_pages	8s
final	3s
	51m 26s

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