build: bump version number to 13.8.1 #2709

Job	Run time
download_test_data	3s
build (macos-latest)	4m 49s
build (ubuntu-latest)	3m 21s
java_matrix	4s
unit_tests (25)	3m 52s
unit_tests (21)	3m 30s
generate_documentation	1m 7s
test_run-groovy	1m 17s
test_clara	3m 47s
test_decoder	3m 16s
spotbugs	7m 3s
test_coatjava (ubuntu-latest, eb-epc)	3m 14s
test_coatjava (ubuntu-latest, eb-ep)	3m 10s
test_coatjava (ubuntu-latest, eb-eg)	3m 58s
test_coatjava (ubuntu-latest, eb-enc)	2m 7s
test_coatjava (ubuntu-latest, eb-eftpi)	2m 5s
test_coatjava (ubuntu-latest, kpp)	8m 1s
test_coatjava (macos-latest, eb-ep, ./run-eb-tests.sh -100 electronproton)	2m 25s
release_build	0s
deploy_web_pages	0s
final	3s
	57m 12s

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