Bump version to 2.8.0dev #1218

Job	Run time
Choose nextflow versions to test against depending on target branch	1s
nf-test (latest-everything, test, docker)	40m 6s
nf-test (latest-everything, test_single, docker)	8m 50s
nf-test (latest-everything, test_fasta, docker)	3m 58s
nf-test (latest-everything, test_failed, docker)	31m 29s
nf-test (latest-everything, test_multi, docker)	18m 54s
nf-test (latest-everything, test_reftaxcustom, docker)	12m 19s
nf-test (latest-everything, test_doubleprimers, docker)	14m 18s
nf-test (latest-everything, test_iontorrent, docker)	9m 30s
nf-test (latest-everything, test_novaseq, docker)	5m 34s
nf-test (latest-everything, test_pacbio_its, docker)	16m 15s
nf-test (latest-everything, test_sintax, docker)	38m 16s
nf-test (latest-everything, test_pplace, docker)	29m 33s
	3h 49m 3s

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