Update go to 1.15

[jbardin]

No description provided.

[codecov]

Codecov Report

Merging #26357 into master will increase coverage by 0.10%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
dag/marshal.go	53.42% <0.00%> (-1.37%)	⬇️
plans/action_string.go	0.00% <0.00%> (ø)
tfdiags/severity_string.go	0.00% <0.00%> (ø)
addrs/resourcemode_string.go	0.00% <0.00%> (ø)
states/objectstatus_string.go	0.00% <0.00%> (ø)
backend/operationtype_string.go	0.00% <0.00%> (ø)
terraform/instancetype_string.go	0.00% <0.00%> (ø)
configs/provisionerwhen_string.go	0.00% <0.00%> (ø)
helper/schema/getsource_string.go	0.00% <0.00%> (ø)
helper/schema/valuetype_string.go	0.00% <0.00%> (ø)
... and 9 more

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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