Bump cdap.plugin.version to 2.13.1-SNAPSHOT #1358

Job	Run time
build (mysql-plugin)	1h 18m 8s
build (mysql-plugin)	1h 18m 8s
build (postgresql-plugin)	48m 34s
build (postgresql-plugin)	48m 34s
build (oracle-plugin)	14m 37s
build (oracle-plugin)	14m 37s
build (mssql-plugin)	1h 36m 54s
build (mssql-plugin)	1h 36m 54s
build (cloudsql-mysql-plugin)	1h 55m 6s
build (cloudsql-mysql-plugin)	1h 55m 6s
build (cloudsql-postgresql-plugin)	51m 41s
build (cloudsql-postgresql-plugin)	51m 41s
	13h 30m 0s

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