Use BaseMigration instead of deprecated AbstractMigration #341

Job	Run time
cs-stan / Coding Standard & Static Analysis	23s
testsuite / testsuite-linux (8.5, sqlite, highest, false)	36s
testsuite / testsuite-linux (8.2, mysql, highest, false)	42s
testsuite / testsuite-linux (8.2, pgsql, highest, false)	29s
testsuite / testsuite-linux (8.4, mysql, highest, false)	41s
testsuite / testsuite-linux (8.2, sqlite, lowest, false)	32s
testsuite / testsuite-linux (8.3, sqlite, highest, false)	35s
	3m 58s

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