Security fix: Update PROPOSALS.md to not reference google groups

[just-at-uber]

What changed?
Removing link to google group that has been compromised.

Why?
Avoid phishing from hackers

How did you test it?

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 0180d8f6-3407-4375-98ac-16498600f205

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 77 unchanged lines in 17 files lost coverage.
• Overall coverage decreased (-0.03%) to 56.903%

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Files with Coverage Reduction	New Missed Lines	%
common/types/shared.go	1	26.66%
common/task/fifoTaskScheduler.go	2	87.63%
common/task/weightedRoundRobinTaskScheduler.go	2	89.12%
service/history/execution/mutable_state_builder.go	2	69.09%
service/history/handler.go	2	47.6%
service/history/queue/timer_queue_processor.go	2	58.37%
service/history/queue/transfer_queue_processor.go	2	56.86%
service/history/task/redispatcher.go	2	89.67%
service/history/task/transfer_active_task_executor.go	2	71.93%
common/types/history.go	4	24.89%

Totals
Change from base Build 0180d8dc-82cd-4f54-9fd0-3c6d40b54354:	-0.03%
Covered Lines:	83948
Relevant Lines:	147528

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💛 - Coveralls