[codecov] Use Go 1.20 as it is specified in go.mod and currently supported Go version on the backend

[3vilhamster]

What changed?
I've set Go version so it will match with our Makefile EXPECTED_GO_VERSION.

Why?
I've seen a log message on codecov execution:

Makefile:50: Caution: you are not using CI's go version. Expected: go1.20, current: go version go1.22.1 linux/amd64

To eliminate possible inconsistencies between executions on buildkite and make and codecov we will use the same version.

How did you test it?
Unit tests on CI.

Potential risks
None

Release notes

Documentation Changes

[codecov]

Codecov Report

Merging #5872 (d570879) into master (9551bfe) will decrease coverage by 0.66%.
The diff coverage is n/a.

❗ Current head d570879 differs from pull request most recent head 8be7408. Consider uploading reports for the commit 8be7408 to get more accurate results

Additional details and impacted files

see 16 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 018eae31-609f-48b0-a34c-de088a3bfc3a

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 44 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+0.003%) to 66.56%

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Files with Coverage Reduction	New Missed Lines	%
common/persistence/sql/sqlplugin/postgres/task.go	2	73.4%
common/persistence/sql/sqlplugin/postgres/db.go	2	80.0%
common/task/fifo_task_scheduler.go	2	84.54%
service/matching/db.go	2	73.23%
service/matching/matcher.go	2	90.72%
common/persistence/historyManager.go	2	66.67%
common/log/tag/tags.go	3	50.18%
service/history/queue/timer_queue_processor_base.go	3	78.64%
service/matching/taskListManager.go	3	80.2%
service/history/task/transfer_standby_task_executor.go	4	87.42%

Totals
Change from base Build 018eae1d-6980-4650-b374-6c5c6277f76b:	0.003%
Covered Lines:	97390
Relevant Lines:	146320

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💛 - Coveralls