change mysql schema folder v57 to v8

[bowenxia]

What changed?
change mysql schema folder v57 to v8

Why?
v57 was upgraded, but the folder name wasn't chnage

How did you test it?
manual test
unit test
integration test

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 01897a32-7678-4010-9050-8d8f80579fa1

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• 153 unchanged lines in 21 files lost coverage.
• Overall coverage decreased (-0.06%) to 57.032%

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Files with Coverage Reduction	New Missed Lines	%
common/cache/lru.go	2	92.2%
common/persistence/nosql/nosqlplugin/cassandra/tasks.go	2	75.57%
common/task/weightedRoundRobinTaskScheduler.go	2	89.12%
service/history/task/transfer_standby_task_executor.go	2	86.63%
common/log/tag/tags.go	3	49.04%
common/persistence/nosql/nosqlplugin/cassandra/workflow.go	3	59.55%
common/persistence/nosql/nosqlTaskStore.go	3	61.07%
common/persistence/statsComputer.go	3	94.64%
common/persistence/serialization/parser.go	4	63.91%
common/persistence/serialization/thrift_decoder.go	4	57.14%

Totals
Change from base Build 01896ff8-7e59-4c67-9bc5-10498582cb62:	-0.06%
Covered Lines:	87114
Relevant Lines:	152746

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💛 - Coveralls