Add changelog for v1.2.8 release

[neil-xie]

What changed?
Add changelog for v1.2.8 release
Release note: https://github.com/uber/cadence/releases/tag/v1.2.8

Why?

How did you test it?

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

Merging #5820 (13f3bb0) into master (b52f1eb) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

❗ Current head 13f3bb0 differs from pull request most recent head 52739f7. Consider uploading reports for the commit 52739f7 to get more accurate results

Additional details and impacted files

see 2 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 018e7c88-18e0-48e5-a7e5-09f8d0e67249

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 81 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-0.01%) to 65.262%

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Files with Coverage Reduction	New Missed Lines	%
service/matching/taskReader.go	2	84.88%
service/matching/taskListManager.go	2	79.7%
service/history/execution/mutable_state_util.go	2	78.13%
service/history/task/fetcher.go	2	85.05%
common/persistence/historyManager.go	2	66.67%
common/persistence/statsComputer.go	3	96.79%
service/history/task/transfer_standby_task_executor.go	4	86.6%
service/history/task/cross_cluster_task_processor.go	8	80.79%
common/persistence/nosql/nosqlplugin/cassandra/workflow_parsing_utils.go	12	85.54%
service/history/execution/mutable_state_task_refresher.go	44	59.81%

Totals
Change from base Build 018e7c84-808b-404f-837f-14737fdbcdbf:	-0.01%
Covered Lines:	95389
Relevant Lines:	146163

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💛 - Coveralls